Age isn't really the issue with buying a car, maturity and common sense is the issue. Judging by your above posts you seem to have thought it out well, and are quite aware of your capabilities as a driver so that in my books would deem you mature enough.
They say life is short, and that's no lie. In my opinion, i would probably go the S15 over the R34 GTT as it has that little bit more class and sexyness (many will disagree with me). But then again, there are allot of other cheaper cars that would take my fancy quite considerably eg, R32 GTS-T/GTR, (im not really a fan of the whales haha) or a nice S14a 200sx. Have a think about it, drive a few of them read lots and either way you'll enjoy it...
YOUR BUYING A NISSAN!!