buy the biggest ones you can afford... that's Charles' recomendation anyhow
That makes for a really nice setup with a lot of versatility at a budget price for sure and what I'd recommend.The GT3788R makes a very forgiving turbo for use as a secondary. It will spool-up pretty fast. I run a GT3788R over a S475/1.31. I start building boost around 1500 rpm. Tons of room for more fuel. I would think you'll have air for 700-800rwhp.
If you throw another ball bearing turbo under a ball bearing turbo like a GT4202R or GT4508R, man that would one wicked set-up