This is my 1.5 cents.
A 50 trim T3/T4 turbo should be more than enough for the power gains you're looking for out of the B20.
A T3 might be too small and would leave you with no room for growth if you build the engine later on.
I would keep the psi low, 4 - 7 psi to start off with. It might sound low, but you'll be surprised when you hit full boost and you don't want to break anything. Also, you could always increase the boost later on, but keep in mind that as said before, the cylinder walls are thinner than the B18's, so they can't take as much abuse.
Every engine is different, but with the PROPER tune and supporting parts, injectors, fuel pump... etc, you could have a reliable boosted, stock B20.
300 hp on a boosted B20 requires sleeves to be 100% safe.
Also, keep in mind the usually people build their engine from hp wanted to boost psi and not the other way around.
From what I can gather, 250hp is at the top end of the boosted stock sleeved B20's engine range.
Hope that helped and good luck in your build / boost.
Happy boosting pimp.