Most of Nasus' damage comes from Fury of the Sands (ultimate) and Spirit Fire, both being magic damage. If you want to see your game improve the most from buying runes, I suggest at least buying Magic Penetration Marks and Flat Health Quintessences. By investing in these runes you'll see your damage output improve and your durability will be improved.
If you're noticing your mana problems from spamming Spirit Fire on waves of creeps, you should think about investing in Mana Regen/lvl seals, or Armor/lvl if you use jungling champions often anyway. There are lots of good choices of glyphs for Nasus, including Flat Cooldown, Mana Regen/lvl, and Magic Resist.
If you don't really care about all of that, some champions you might enjoy are Udyr, Rammus, Gragas, Jax, Mordekaiser, Olaf, Poppy, Singed, Sion, Vladimir, or Xin Zhao.
Hope this helps and remember to have fun!