I like nasus and I feel like I do well with him (if I say I am good I'll curse myself to lose the next 10 games with him, so no I am not good ).
I like to build him as pure tank, with lots of regen. I agree with philo stone first item, and max q first for easy last hits. I used to make tri-force regularly, but I've decided it's not really needed, you do pretty crazy damage with ultimate + Q anyway, and building the triforce puts your tankyness on hold for too long. Merc treads are usually the most appropriate boots, unless the enemy team suggest dodge boots. I like a sunfire cape, fits with his ultimate and just gives you another reason to get right in the middle of everything. Beyond that I build to the enemy team, force of nature + banshee for casters, warmogs for true damage, thornmail for excessive ad carries. I usually don't build any real damage items, but I'd build a ghost blade if the game really called for it, just to make him shred towers a little harder.
Tower kill is his main role IMO, but he also allows for very early and quick baron kills. The way his ultimate works you can take baron down to half hp or less just from your ultimate.
I'm not a huge fan of Sunfire. The 45 damage per second is reduced by MR significantly. It is only good if you rush it early, because it is an early/mid game item. Late game it's damage output for the cost is bleh. Sure you get some defense and life from it as well, but the combine cost isn't worth the cost of putting it together late game. That and it requires you to be right on top of people. Nasus can do that at least with wither, but not every hero can. The only three heroes I consider putting it on is Amumu, Malphite, and Warwick. Warwick because you should be getting madreds + malady to actually make sunfire worth while late game when you use your ult.
I like the bruiser build I listed above (duh because I listed it) mainly because it still gives you good health, defense, and dps in a few items. Plus gold generation. The cost isn't that high to get into either for the returns.
If you decide to build extra tanky completing the HoG to Randiun and grabbing a banshee will put you at around 250~ish armor and 180 MR. With tri-force + randiun + Reverie (once you complete philo stone) that puts any hero over 3K health. If the other team has a lot of true damage, like vayne and ignites, then dropping the philo stone for a warmogs + atmas will allow you to be super tanky and have ridiculous damage output late game. Of course that is a lot of gold to get there, but Nasus is capable of that.
Personally the ultimate Nasus build, and pretty much the ultimate AD bruiser/offtank build, is the following items.
Merc Treads
Aegis Of Legion
Tri-Force
Atmas
Warmog
Last item is based off enemy team. Either Randuins or Banshee Veil depending if they are more AD or AP. Or last but not least is GA if they are a mix.
For most AD bruiser chars these items puts them around 4K hitpoints , 300 AD, 1.3 attack speed (with attackspeed quints), ~100 MR, ~100 armor, 420 movespeed, 35 tenacity, and 39% crit rate. Plus all the nifty effects from tri-force, aegis, and warmogs. That's without taking into consideration any of the abilities or passives of the champion being played or the 6th item to be picked. Once you go past 100 armor or MR you start getting massive depreciating return value per point. 100 in either is 50% reduction. 200 is 64%. The only reason to go higher is if the other team has items or abilities to reduce your defensive values. Which a lot of enemies do so getting either value higher based on who you are fighting with that 6th item is usually a must.
However that "ultimate" build I just posted is not exactly practical, nor easy to gain. It takes a ton of gold to reach that level so unless the character you are playing is a massive farmer or can get fed pretty hard, it isn't going to happen. Also most game are over FAR before you can complete the build.
Just for reference, for most bruiser champs I used the item builder for with this build it puts them at an effective 9-11K hitpoints (meaning the enemy team has to do around that much damage in total to kill you), with around 650+ dps. It's pretty disgusting actually.