Zilean's kit doesn't allow for a lot of variance. His Q has a 90% AP ratio on it, and his QWQ combo is extremely easy to spam -- not building lots of AP items is a ludicrous notion, and it's just a matter of where Zilean gets his mana (and how tanky you feel you need to make him as you build your AP. There are AP items like Zhonya's Hourglass, Banshee's Veil, Hextech Protobelt, etc etc which can help Zilean survive a bit better. I don't recommend them, because if you need them then you are playing Zilean too far forward.)

I do have my own twist on Zilean in this build though; the inclusion of Nashtor's Tooth. It can be pretty easy for Zilean to stun an enemy champion with his QWQ combo, but doing significant damage beyond that can be difficult for most Zilean builds. Generally players are content to land the occasional Arcane Comet, and they call that "good enough". I like going for more. Nashtor's Tooth + Hail of Blades allows Zilean to do some good auto-attack damage; either after his QWQ combo, or before it since opponents can't stand and trade with Zilean without risking taking his combo. It really only takes completion of Archangel's Staff for Zilean's bombs to start really stinging. By the time he finishes his second item (Nashtor's Tooth), he can kill squishy champions in two combos. Zilean only spirals out of control afterwards; the other buys are Sorcerer's Shoes and Void Staff for MPen, and Rabadon's Deathcap.

Definitely it is. The build, other than Nashtor's Tooth and Hail of Blades to make the Nashtor's Tooth shine a little brighter, isn't remotely new. What is not common is Zilean in the support role, but he does actually shine there in some ways. Basically all that I did was take my Zilean mid build and toss it into the support position to see how he did there.

First, Zilean is excellent at zoning off the enemy by tossing out bombs. This allows your ADC to pick up farm much more safely. Diving into Zilean's bombs easily leads to getting stunned and killed, so the enemy has not much say in the matter.

Second, Zilean can potentially stun BOTH enemy champions at once. This makes getting kills/assists for your ADC much easier, since it isn't just the low health target that is unable to go after your ADC. The enemy duo will try to fan out to avoid Zilean catching both of them at once, but that means they can't really get to your ADC at the same moment easily.

Third, Zilean can rapidly shove the lane. It is worth "stealing" some farm to get tower plates. Since Zilean pokes well, you get into situations where you can force the enemy duo out of lane and go for the tower right in front of them. If they contest, they take a QWQ and might die from it. You do have to be on point with placing river wards if you do this, as the enemy jungler will not like you winning bot-lane this hard.

Let's talk Tear of the Goddess, an oft maligned item. So much gold for no stats, oh no! But mana is a stat. You get to see how much damage extra mana does whenever you run out of mana and suddenly have no buttons to push.

Rush Archangel's Staff on Zilean, EVERY time. But I want to break down the component items here. Buy x2 Sapphire Crystal first, before you complete Tear of the Goddess. You will stack up Archangel's Staff quickly on Zilean; you don't have to get it before you get the Sapphire Crystal that will be part of Lost Chapter. Getting an extra 500 mana on your first back is huge for Zilean. You will never run out of mana after that moment. On your second back, get Tear of the Goddess finished and buy both Tomes if you can. Basic boots are OK too.

Once you have your Archangel's Staff, it's on to getting the rest of your CDR. You want to go Fiendish Codex right away, to ramp your bomb damage/rate further. And here is where you have to make your first in-game decision. First, look at your level. Will you get 1100 more gold well before hitting level 10? If so, get Stinger on your next back. If not, it's best to start going into MPen or Deathcap; Nashtor's is to augment your bombs, not replace them. But I admit that I almost always go ahead and build Nashtor's Tooth, because I am lazy about it and prefer to spend my mental energy on the positional battles going in in lane. It's 100% acceptable I think to operate that way, since with Transcendence Nashtor's is actually giving you 100 AP not 80. It's a very good AP item and you do greatly augment your total damage (and improve your aa poke damage a lot).

Sorcerer's Shoes are a priority after CDR, just to stay safe and get around. You can, since you are going to build Void Staff, go for other boots. Zilean's damage ramps so high that you don't "need" Sorcerer's Shoes to be deadly. But if you do get Ninja Tabis, Mercury's Treads, or Boots of Swiftness to stay safe... go Void Staff before Deathcap. You will need its MPen more if you lack Sorc Shoes.

Let's talk Hextech Gunblade for a moment. It's the item I think is best in place of the support item; something you would build at some point if you were going top/mid with this build, or something you might build late into a game even as support to help close it out. You want it for healing up against being whittled down and then all-in'd. If you are taking a lot of counter-poke, this is the right item to build after completing Nashtor's Tooth. You'll be stronger poke vs poke, and you'll be much stronger if you get all-in'd between QWQ combo attempts.

Hail of Blades is necessary for this build, to go with Nashtor's Tooth.
Cheap Shot is OK-ish. You only get it when you land a QWQ combo.
Zombie Ward is OK-ish. I hate face checking brush with squishies tho.
Ultimate Hunter is OK-ish. I usually take it, even tho it's weaker here.
Transcendence is +20 AP here. It could be Gathering Storm if you want.
Manaflow Band can also be Gathering Storm. I prefer Gathering Storm here.

If you take Ultimate Hunter, you can consider a trick; die to something on purpose in order to heal up. If you choose a jungle monster (Dragon is best here), the enemy team won't know that your R is on cooldown. Your R will come off of cooldown pretty quickly, allowing you to stay out on the map. It's risky but can pay off. Think of it like getting all that health NOW instead of later as the enemy team collapses on your position.

Cheap Shot is excellent if you are supporting Ashe. You'll be able to rake in the extra true damage much more often, and it becomes easily better than Taste of Blood. I personally still take Taste of Blood, because I don't necessarily trust my Ashe to be alive and poking at the enemy. If you duo with an Ashe you know plays aggressively, definitely you can make a different decision though.

WHEN TO TOSS OUT THE QWQ
There are mainly two ways to play Zilean. You can either toss out a single Q and wait, or you can commit to a rapid QWQ combo every time you hit Q. Both have merit.

The single Q is for poke damage only, to get targets low when you know they won't run away. It allows you to conserve a second bomb (WQ) if the bomb misses. When some poke damage is enough bay the enemy, this is a reasonable tactic. But if you NEED access to your stun to keep the enemy off of your ADC, you can't play Zilean this way. Use the single Q in lanes where the enemy duo don't heal and don't want to all-in you. Something like Jinx + Braum comes to mind.

The QWQ stun combo is for getting kills and warding your ADC against hyper-aggression. You can usually tell when you are in such a lane. You might be against Vayne + Nautilus or Xayah + Leona, duos who are just itching to all-in you or your ADC. Here, you have to go for QWQ. If you land it, you and your ADC can opt to follow up. If you miss, your ADC knows to back off while your cooldowns reset. Going for QWQ is a way of communicating to your ADC.

HOW TO TOSS OUT THE QWQ
There are two speeds at which you can toss out a QWQ combo:

If you QWQ as fast as you possibly can, you have the best chance of landing a stun. If the first bomb hits, the second almost always will too. The chance that you take here is that you miss both bombs and don't do any damage. When trying specifically for the stun, do not hesitate once you hit the first Q.

If you Q then wait, you can see if the first bomb hit. If it did, you can try to WQ and land the second bomb to get a stun. At lower levels when Q is on a shorter cooldown than W, this can be a good trick to have your QWQ combo available more often. But once you have 4 ranks in W, stop using your bombs this way. At this point the cooldown for W isn't longer than Q. You can just QWQ instantly and get both buttons back at the same time.

Of note here is the skill rank order section. Note that after getting 4 ranks in W, I say to wait until level 18 to get rank 5 in W. This is because you do not need W's cooldown to be lower than Q's cooldown. The purpose of rank 5 in W is to maximize how often E is up, which is extremely low priority.

Zilean is my favorite mage, really the only one I play much of than Ziggs. I have a ton of games on Zilean and often choose him when I think I might be heading into a 4v5 game where I will be the one likely to face a 1v2. He has huge carry potential, and he takes off as a serious threat well before the enemy team could start pushing for a win. I've built him a lot of ways, but this way always feels the best by far.