from what i've heard a new intercooler isn't necessary until you start getting some real numbers, like aroun 320 at the crank. This was via a turbo xs faq I read awhile back. The vf22 is a nice turbo. It's bb and has really good top end. However I don't know where you'll get one for 750, mine new was 1,000 and that was cheap from what i've seen them go for. I would say go for this....uppipe, downpipe, exhaust. and then get I think ECUtek and have mastro tune it on their dyno. I'm not sure what ecu's they tune, but it's either ecutek or something like that. With all that done you will have enough $$$ left over for a lightened flywheel and a new clutch. I know the flywheel was probably the most responsive mod i've ever bought for my car.
or you could go the another route and get full exhaust, fuel pump, injectors and some nitrous.
there's tons of options, just look around on the impreza forums for info on what ppl have blown their motors running and you'll get an idea on what NOT to do. Have fun!