If you stay low medium in the light category you might get away with just running DIY CO2. 30 gal seems to be the accepted max size for the conventional DIY set up. Why not set up a pressurized system? A light fixture to put you in the "high" light category is nearly as expensive. With pressurized CO2 you could get into the medium/high light range and still effectively control algae. You'll never regret CO2. You likely will regret constructing a small sun on top of your tank.
It's been my experience that anything above medium light not only requires CO2, but really good, and well distributed CO2.
I don't think an XP2 would be too big at all. It's probably just about right.