Nasus on Top Lane is very powerful Off Tank. Can be week in early but if you farm your Siphoning Strike can be most powerful champion in hole League Of Legends. In early game your Primary Goal is to farm your Siphoning Strike. You need to play aggressive so your opponent play defensive and you can just farm or to freeze lane. Your Siphoning Strike should be 250-300 in 20 min. If you got more even better but that's the number your should have in mind to have in 20 min. When you play Nasus try not to use many of mana your on Wither and Spirit Fire for nothing. Sometimes opponent can bluff and make you use your mana for nothing so be careful.
For me better is to buy Trinity Force then Ice Gauntlet because with Boots of Swiftness and Trinity Force you don't need the slow of Ice Gauntlet because you move with those 2 items much better and you have Wither to slow and later very small CD of Frozen Heart and Spirit Visage and Trinity Force is better because you get HP, mana, MS, AS. With this Nasus build you have damage with your farmed Siphoning Strike, HP, Armor, MR, 30% CD, MS and you can revive your self every 5 min with your Guardian Angel

Nasus in Team Fights need to tank for his team. With this build Nasus has high Armor and MG and has HP and can get more HP with ultimate. Nasus can take damage tank for his team but still can make a Big Damage especially if you farmed your Siphoning Strike. He can slow enemy with his ]Wither if they are on the run. But most important his Spirit Fire is very useful in Team Fights because he reduce their Armor and MG. So your team of Spirit Fire can make more damage and kill them. If Nasus has farmed Siphoning Strike he can kill easy their ADC and Mage or make them on the run and kill then the other enemy champions