Feel free to make multiple rune pages, for example magic resist per level works fine too if your top and jungle opponents are both ad.
Lifesteal makes your sustain superior, you have never too much lifesteal with Nasus and lifesteal basically scales to end game too, the more you hit the more you get health back!
Attack damage marks helps you to last hit with your Q and basic attacks, you can also change these to armor if you are against Riven or other ad heavy champion.

I go always ghost teleport because you have to help your team. With teleport you can solo dragon when you are level 6, you can splitpush and teleport back to lane when you need to recall to maximize your Q farming. Ghost helps to catch people and flash got a way too long cooldown.

Still need to think more about masteries. Defensive masteries are currently very strong choice and there is probably a nerf coming on their way.
From offense the cdr is brilliant before your core build to maximize Q farm and other masteries help you last hit and helps with auto attack trades with your opponent. But if you think you can last hit easily without the bonus ad why not 0-21-9 to get the sweet biscuits. That 5% cdr is useless end game if you end up buying Iceborn Gauntlet, Spirit Visage and Frozen Heart. Offensive masteries are mostly to secure early game last hits.

Max Q to get less cdr and then E to get a great amount of armor reduction. W isn't so necessary early, 1 is usually enough to escape from a gank but putting more points to it later can also be benetificial especially against champions that rely on their attack speed.

From personal knowledge I can safely say that the hardest matchups against Nasus are:

Elise: Keeps poking you with his abilities and auto-attacks. Extremely hard to farm against early.

Kennen: Keeps auto-attacking and poking you making your early game hard. Do not try to get every creep. When you get a little magic resists he becomes a bee.

Lissandra: She can spam like crazy and make your early hard. Usually pushes the lane with Q spam so you can farm under turret.

Pros
+Farms well
+One of the strongest late game champions
+Scales extremely well
+Great at killing PvE objectives

Cons
-Never really pressues enemy champions in lane
-A lot of counters to Nasus
-Kited very easily
-Farm reliant

Your goal in lane is to farm and nothing else. By picking Nasus, you've already made a commitment to your team that you will be a a liability in the early game only to come back in the lategame to wreck faces. Therefore, stacking up your Siphoning Strike is your primary objective. The only instance where you should be using your Spirit Fire is to prepare the minion wave so that you can reliable last hit at tower. And even in that case, Nasus is one of the best farmers under tower because of his auto attack reset and the lifesteal from his passive allows sustain.

You will lose lane a majority of the time but if you play safe and farm your Q, there are only a few scenarios where you'll find yourself on the losing side if time allows.

Strengths
Marksman lockdown and lategame monster.

Throughout all stages of the game, as long as you have Wither up, you are a Marksman's nightmare. The MS and AS slow is one of the strongest in the game and is only countered by burning Cleanse or purchasing a Quicksilver Sash. Be sure to coordinate with the rest of your time not to stack CC on targeted carries so you can get the most bang for your buck.

Past laning phase and during teamfights, you'll become one of the strongest champions in the games since Nasus scales with time so well. With a skill that scales literally forever and 20% lifesteal past level 11, both your damage and sustain is unrivaled. If you were building health and resistances, it will literally require an entire team to bring you down while you murder squishier targets.

Why Pick Nasus?
If you have the patience to farm Siphoning Strike for 20+ minutes and be subject to the whims of the enemy team during that timer period only to come out as one of the strongest champions in the game, you should pick Nasus. Lategame, you can literally 1-shot carries, take down turrets quicker than marksmen, and bring down Baron and Dragon at an insane clip.

Tips and Tricks

Play safe and farm for the early game to wreck later on.
Use your Spirit Fire to prep the minion wave for easier farming.
Coordinate objectives such as tower sieges and monster control with your ultimate since it allows both tankiness and damage.
Fight in Spirit Fire as much as possible for the armor debuff.