No point going high mount, use your standard manifold with a S2 RB25 turbo + elbow. You can retain all your current lines, dump/exhaust and intercooler piping etc as its straight bolt on
Get the fuel pump in and get the ECU remaped
You should see 180rwkw + but that won't get you to 200rwkw
Its been done a million times
If I was going for a responsive 200rwkw I would go with a HKS2510 or maybe even a HKS2530, even an RB25 high flow but not too huge - all those will blow your budget of $600