AP Off-tank Singed player here.
I like his passive as it is, although it could use something to spice it up, as he benefits a lot from items like:

Mana is good for singed. Great for him. Especially from items like RoA and RG. Why? Because he gets a nice chunk of bonus HP with other useful stats as well. Whether lategame or laning, you want enough mana to be able to harass people with flings, slows, and roots into chasing you.
Both RoA and Righteous Glory are worth the gold. Because you get sustainability and damage from RoA, while you get a nice initiation tool and sustainability from Righteous glory.
My build tends to go along the lines of:
in most cases, but if my team lacks initiation I take
instead of 
A bit squishy, but the aggro it generates is insane.