The BC Exhausts are crap is a bit Genreic James. I had the Spec IV and thought it was an amazing system, well built and amazingly priced, and not too common so you wont see them on every car like the Spoon N1.
Cane - Another alternative for intake is the AEM CAI, very effective and about a 1/4 the price of a Gruppe M. If you want ultimate JDM points, start looking at FEELS exhausts, very rare on the scene at the moment. As people have said a FRSU is the first thin to go for, it will totally change the car and the way it drives. Going for lighter rims is a good start too, lose some of that rolling mass. If your budget is quite large, as you said, then the opportunities to go nuts on the K20 are there. Look at "G's" car, he is a member on here who has done some extreme things to his engine and stuck with it being N/A. It is a good example of what can be done with these engines without adding turbos' or chargers.
One last thing, keep us updated with pics mate, we love a build thread around here :lol::