Warmogs combined with spirit visage gives u endless sustain in team fights with the new patches even giving it CDR which is perfect for Soraka.
Mejas is kinda useless since Soraka has really ****ty ap scaling. Your agrument is that if u get ap it will heal you more through your Q (Q healing sclaes with 10% ap). Even with a full 25 stacked Mejas that gives you only 14.5 extra healing per correctly landed Q.
If you have warmogs + Spirit visage and just as an example 3000 life (bare minimum). That gives u 112.5 HP every second! And the Warmogs passive does not get canceled when u are just healing.
Furthermore, you´re a huge target for the enemy team, so building squishy ap will make killing you very easy. Plus mejas only gives the enemy team another reason to focus you.

I would like to point out something about Warmogs that isn't being taken into account. You're banking on Soraka having to leave battle to activate the passive but this isn't necessarily the case. When I play raka, i don't just stack up hp indefinitely. With my runes, and the items that I build on her (sightsonte rylais warmogs + a ruby crystal for whatever it needs to go to) at lv 14 soraka has the 3k hp required for said passive. Now if raka is IN COMBAT then yes, she doesn't recieve the passive. However, the passive does continue to heal her as long as she is OUT OF COMBAT. Healing allies isn't combat, nor is it counted as combat, nor does it stop warmogs passive from being in effect. this means that during the fight, while you are healing from the back line, you are constantly being healed without the need to even use your q. If your team gets into a really bad spot, you can drop your silence and use q's to try to help the team kite out as both your q and your silence (thanks rylais) is slowing the enemy. This makes the stuns more likely, and your q's easier to land till you can return to the safty of warmogs all while healing your team. Now, i would not suggest this kind of play style against, say, a team with assassins like fiz/talon/zed/master yi/etc. however i would suggest it against teams with tanky engagers who have situational cc, Shyvana/Udyr/tahm/etc. Cc that you can kite basically, because thats what raka is now, the kite queen. Obviously one of the two remaining spots will be boots, i usually upgrade the sightstone into my gold income item so i have 2 left after boots if the game looks long enoug for 6 slot. These are the items that i grab for ap, usually i'll grab like zeke's and or ardent/grail. Anyway, just something to think about. I don't always go off-tank raka, however I don't think it's a sin. While your team brawls it out, you heal for almost all of your w's 10%hp from warmogs passive... almost. and when everyone is at about half hp, you get it all back anyway while you lay off the heals for a couple secconds.

I also find it unfathomable that people buy health.

Hi, NightsBane!

Actually, you can disrupt Tahm Kench. The most dangerous situation your AD carry can get into is to get swallowed, but if you see that Tahm landed his Tongue Lash on your ally and going to auto-attack, place Equinox under ally's feet. That leaves Tahm with three options: turn back, try to maneuver while being attacked by you and your ally or try to cross the silence zone. The last option may result in overextending, which means Tahm's AD carry either isn't able to catch up or loses his farm.

I don't really like building Mejai's Soulstealer on Soraka because she already has big *** red bold mark over her head that says 'I'm Soraka, if you don't kill me, no one in my team dies'. If your enemy is smart, they'll do everything to shut you down. And Mejai reminds them about that and gives even more inspiration to do so. That may lead to some deaths among enemies, but you'll be left with pretty much useless item that costed you 1400g. I'm not saying I'd never buy it, it's a decent item, in some matchups with immobile enemy team composition and your team ahead it may do wonders. But in general, I would not recommend it, at least not as a core item, because it's way too situational. Although, it may work better in lower leagues.

I feel like I should remind you that you didn't add some runes to your build, it's missing marks and glyphs.

Good luck with expanding your guide!