No internal work is needed thats correct
the kit is meant to be fitted in about eight hours, but i would leave a weekend
the vr6 is meant to be good for about 400 bhp without major internal mods
there are many threads on the vr6 owners club and corrado forum (use search) and just google the forum names that should breing them up
lots of discussion on the best kits avaliable etc due to differences in belts used, software and chargewr bearings etc
hope this helps
Tim