I play a lot of Nasus and follow Hyfe's guide pretty closely for most games. I tend to start out with meki pendant with 2 health pots and build Aegis, merc treads, then at least a negatron and/or a chainmail before sheen. I like to ask my team after I complete aegis if they want me to be tankier or more dps, then build situationally from there. Their assessment helps me decide which way I need to build for endgame
If you are going for dps, get sheen, then triforce. If your team doesn't have a starks and there is at least one other autoattacker on your team, get starks. If you don't need starks, I go for phantom dancer for movement speed. Then build defensive items situationally against your opponents. 15 kills for opponent malzahar? build FoN. Opponents twitch is carrying instead? Buy thornmail or randuins.
If tank I generally get aegis and merc treads first, negatron, heart of gold, finish FoN, finish Randuins. This is most common thing for me, but I don't think you can just go into a game saying "I will buy meki pendant, aegis, merc, negatron, HoG, FoN, Randuinds"...that is just the most common thing that happens for me A few situational clues: If you are under-farmed get sunfire instead of randuins. If they have 3 or more auto-attackers get thornmail after negatron....
The only time you should get chalice first is if you are in a 1v2 lane or mid and are being zoned. In those cases it is in your best interest to get one level in q and w and then spam your e enough to keep the opponents creep off of your tower, but not enough to have your creep at their tower
Runes: movement quints, mana regen yellows, MR blues (I have run mana regen blues before too...this is basically a free spot that you can put anything you really like) and armor pen reds
Masteries: I like to get the mana regen and movement out of utility tree and all of the CDR stuff so I generally go 9/0/21 making sure to get exhaust from offensive tree and the things I mentioned above.
Summoner spells: I like teleport and exhaust. Once your q is farmed up you can utterly destroy towers in 10 seconds, so teleport is really handy (same reason you will see heimerdingers and master yi's taking teleport). Exhaust is good because it lets you shut opponents and keep them close enough for you to thwock and AOE their backs as they run away. Chain exhaust with wither, or in a big team fight you can wither physical dps #1 and exhaust physical carry #2, pop ult, E on top of where they will run away, and Q their faces in. I sometimes run ghost or fortify, but teleport and exhaust are really where it is at with Nasus