Now that's a silly question....How much power do you want? 400+ is possible out of a reliable VQ but with a spicy cam that might be a little rough for road use. 450-500 with boost until you start fitting forged everything inside.
No answer to it really, enough to make it work, then if everything works, more and more is the answer I guess.
My thinking is NA to start, but the choice needs to be boostable without grenading itself, which to be fair is true of most modern v6es. Jaguar (plenty about), Honda (fair few about), Nissan VQ(ditto) all fit the bill. Trouble is I can't get the kv6 out of my system, having done tens of thousands (hundreds actually) of miles with one. It would need a turbo though, since it comes in 2.5l max..
Wish I worked in a scrapyard....