Either way you go; Porsche 6, or big/turbo Type 4 motor, you're gonna spend some long dollars. It really depends on what your budget is. All I can say is, if it were me, I would stick with the T4. Those sixes are REALLY heavy, and any performance/maintinence upgrades are going to be expensive.
As for what HP and torque you want.... Well, there is ALWAYS something faster. I'm not sure exactly what answers you're looking for here, but I'll offer this. Being that the whole basis of the car is it's light weight/reletively low power design, any small thing you do will make more of a difference than it will in the average car. So, lightening the car up a few pounds, and making the extra effort to make sure your engine is tuned just right to get everthing you can out of it (whatever engine you decide to run) will make a big difference. Depending on your personal build, you may even consider going on a diet to drop those 0-60 times.
Seriously though, 0-60 comes a lot easier with a lightweigh car, as opposed to a really heavy, really powerful car. I'm rambling now because I've been awake for way too long, but hopefully I gave you somethings to think about.
Mark D.