Good spec mate! Easy 200bhp if your metering unit can fuel for it, or as you say slap a 5th injector in there.

A couple of things i'd do differently...

Standard Ford/Payen head gasket (good for 350bhp proven!) Felpro etc. are a waste of money.
Stage 2 head, a Stage 3 big valve will actually slow you down (laggy) because of the drop in gas velocity caused by having bigger vavles/ports! If you want to go over 240bhp then go big valve.
Newmans Cam & Solid lifters, hydraulic setups are a lottery now a days but feel free to play if you want lol
Low comp pistons, will lose you more off boost power and for your target BHP they are not needed, stick with the stock C/R!
T25 BB, I dont know much about what they can flow but just be sure it can do enough for what you want.

Also what boost you end up running will be determined by the volumetric efficiency of your engine (your head, cam, exhaust, air filter, and turbo effect this) and your target BHP, it's not something you can choose as such (unless you just decide to cap it for what ever reason?!).

Look forward to hearing how you get on mate!