Is Singed mana-hungry?

His mana pool also affects his hps, a ROA or Tear is usually taken to compensate his mana needs, i'd tend to lean on Roa over tear tho.

He starts off mana hungry, but ends up super fed

[slayer-jinx-wink]

A little bit in early game if you mismanaged his poison and fling. Fling is very expensive.

Mega-adhesive costs 70-110 mana, Fling costs 100-140 mana and Insanity Potion costs 150 mana. Combined with the consistent cost of his Q, his mana costs can be rather high.

However, you should not be using W or E too frequently when playing him properly and his ultimate provides mana regeneration over the duration.

You should probably be able to get away with either two Doran's Rings or a Sapphire Crystal (possibly a Tear of the Goddess) for mana sustain.

I would not recommend stacking mana (i.e. Rod of Ages and Seraph's Embrace and Frozen Heart), since it's rather inefficient for survivability and he doesn't need that much mana.