Cause that'* the rules! LOL
A GT3076R would work on your engine, but I would go bigger. A GT3076R turbine side is really small because it uses a T2 flange. This makes for a quick spool, but limits the amount of boost. I wouldn't do anything larger than a T4, but it really depends on your horsepower goal. If you want to do cheap, than try to keep the horsepower under 400. A T3 should be able to handle that.
I like internally wastegated turbos. That saves tons of money and exhaust work. I think the 3076R is the largest you can get with an internal wastegate. EDIT: GT3271 http://www.turbobygarrett.com/turbob...1_452203_1.htm