550bhp is going to be nocking on the door of £20k from a standard car.
So it's a case of how deep are his pockets?
Brakes are !Removed! on early one so thats a must, 330mm AP 6 pot kit is £1250 from godspeed, if i were doing brakes thats where my money would go, you can get cheaper kits, but AP's are the daddy and if your eventual goals are 500+ then you need proper stoppers.
Engine wise depends how lucky he is, some early ones actualy had closed deck blocks. WRX's were kinda throw together with what ever they had on the shelf, some folks got lucky.
Has he got a budget in mind he wants to spend?