Because of his mana costs you'd absolutely not want to rely on Spellthief's for gold, Relic Shield being great since that would let you support your teammate via healing while you stack. AP lets you be a factor when you can't get in melee range which is even more important since you have no hard cc and sub-par engage.
You will be able to get rid of your Relic Shield once the game gets late enough, but outside of your gold item, boots (buy your pair of choice based on the situation), and sightstone, you have a couple of options for your last 3 items (4 post selling gold item).
Option 1: Liandry's, Rylai's, Lich Bane (, Luden's). This lets you E and R damage deal fair damage against all opponents and makes your E and R AoE slows. Lich Bane doubles down on the damage from your stacks and let's you get some mana to help for those crucial late game fights. (Luden's adds to burst, increases the damage of Lich Bane, and gives you mobility to manage battles with more options)
Option 2: Zhonya's, Luden's, Rabbadon's (, Lich Bane). This simply lets you burst people down with as much AP as someone with a sightstone can get.
Option 3: Rod of Ages, Abyssal Scepter, Zhonya's (, Lich Bane). Grants you more mana than the other options to let you spam with more freedom and grants you more defensive stats than the other options. You may have a tough time getting the RoA stacks if you get it too late, but this would let you be useful longer in fights and use your abilities more in fights if you can pull it off.
All of this said, I'd still prefer a tank Nasus support since he'd synergize better with Relic Shield and Sightstone while transitioning to the same late-game role that standard Nasus top has. You'd basically be sacrificing your laning phase unless jungle pressure swings heavily in your favor, but this would be the most consistent form of Nasus support and the one that would take the least amount of time to get good use out of.
