I have been playing nasus alot lately and i freakin love him, this is typacially the build i do in games. Usually they have high ap and not much ad. tri force,fon, and thornmail or sunfire capes are usually my core build to make myself the tank of tanks. then if a game goes to long the last two items are reasurance since when your in your ulti for a team fight its almost impossible to take you down resonably in a team fight.
Having a very defensive build requires you to farm your siphoning strike very well so you can scale into late game and keep a nasty 3 sec nuke. Always aim for the squishy carries to keep them running so your team can wreck!!

with nasus i have found that in the laning phase playing very passive cassualy farming Q until the enemy players make a move or a mistake that you can easily capitalize on. toward the end of the lane phase you will have your ulti and be able to wreck some serious dmg on the other team with your slow keeping them close. after the lane phase i run in and out of lanes pushing back ther minions and when i get a chance take out the jungle. The only problem with this is that he is mana dependant so blue is allways good to have.
Try to only use your Q when farming or cleaning up a lane your spirit fire takes way to much mana for wht it does until a team fight or protecting a tower.
Wht is very nice is there is no need to life steal since your ulti will boost your regen and your passive starts at 12% and goes up to 20% bass life steal. so you can go ful tank but be a force to be wreckoned with as a dps.