I would just choose a low mileage G and supercharge it. How long before you can get a nice G35 sedan for under $20k? Maybe another 2 years.
I am sure that there will be more forced induction options for the G35 within a year or two, since it shares the platform with the Z.
The Stillen offering for the Z can increase power to above 350 bhp. That would be more than sufficient for a car like the G.
Think about it , an excellent, reliable car with 0-60 in the 5 second range and a price tag of < $25 grand.
As far as reliability is concerned, most forced induction options place very little stress on the engine when there is a light load. Only when you stomp on it, does the boost put some stress on the engine. Even then, it would only be for a few seconds - max. 15 sec.
As long as the kit is well-engineered and the car is in the proper state of tune, there should be no problems, except for the egos of other drivers.
-sijoko
