Subaru Impreza GC8 & RS Forum: RS25.com would probably be a good place to start.
What manifolds? You never said anything about different manifolds. Most different intake and exhaust manifolds should fit. Pretty much everything on these cars is interchangeable.
The thing is, the engine in the 2004 2.5RS is an ej251, not the ej205 found in the 02-05 WRX. The ej251 is a single overhead cam engine with 10:1 compression ratio. It is an open deck case and not built to withstand boost. I have never looked much into boosting a non-turbo engine (mainly because you would be better off just replacing the whole thing), but generally you can run up to about 5psi without blowing it up right away. It still wouldn't be reliable, and like I already stated, you don't have the wiring or ECU required to run and tune a turbo charged engine.
If you're intent on turbo charging the most sensible thing to do would be to sell it and buy an actual WRX. Second option would be to replace the entire engine/wiring/ECU with one out of a wrecked WRX. Third option would be to continue down the road you're on.. I'm sure there are threads on rs25 about boosting, but this will most likely end up being the most expensive and least reliable option in the end.