The S4 is a nice car, but it'll take quite a bit of work to it to get it to perform like a Z. You're better off buying a much cheaper, more reliable daily driver like a used Toyota corolla and spend the extra money on the Z to make it even faster.
: You can get an S4 and do a stage III on and get into the 13s which is not as great 300ZX but still nice.. Also if you get some upgraded turbos then you get some real power..