sf-fc3s

I got an 88 T2 and I wanna build her as strong as legally possible. I'm trying to decide if I should do an s5 swap with wire harness motor and the works or build up a new s4 motor. I don't know which is a better base motor. I know that there's that damn flapper problem with the turbo mani on the s4 but plan on runnin' a Turboblown long runner and a T04Z on it. My family at home is plannin' on sending me parts and I dunno whether they should send me s5 parts or s4 because I don't know if I should save a bunch of s5 parts in the garage and wait to get a motor, or go ahead and put some new s4 parts in and prepare for a new s4 motor? What's cheaper and more practical I guess? I'm trying to keep it on a fairly normal budget and without causing too much problems in California when I head back home. I guess the main question is, is the s4 a strong enough base for some serious horsepower? And should I go jspec or US? Keep in mind CA legal. Any advice would be appreciated.