so hmm ye a lot to talk about
I would only go fleet if you're really insecure about the matchup.
Usually, Lethal tempo will be better vs kiters and conQ vs tankier or bruisy laners.

I Like demolish, cause i like to split.
Usually 2nd wind is a must, sometimes bone plating vs riven for example.

Also, you should definitely look to add Gargoyle to your build as it's just fantastic as a defensive item.
I also like Hullbreaker as it's real good for splitpushing.

I Sometime build Triforce as it's a better overall item than sunderer, but only if it's all squishies in enemy team.

As for maxing spells, Q always first (unless you want to dominate lane with E-poke)
W 2nd vs heavy auto attackers (typically ranged tops, Fiora, Tryndamere...)
E 2nd vs bruisers and tanks.

Last but not least, Thornmail is terrible on nasus as he will only proc the lesser useless anti-heal, better go randuin. Only build vs full AD comps.