A cars age isn't one of my criteria. If it performs equal to/better than a Z, is more comfortable, quieter, for similar investment, it's a candidate for consideration.
My E36 lacks some tactile qualities. It's somewhat boring to drive, but very capable. I think there is some latitude to trade off some comfort for tact, and still have an easy car to live with. I paid less for it than a typical used modern Honda.
My departed E30 was the closest thing to a 240 I've ever driven in terms of feedback and total performance, but it was light years more comfortable, quieter, and refined. It had a bit of suspension work, but nothing that kept me from driving it regularly, or on longer trips. I'm even willing to say it handled better overall than any street-able S30 I've ever driven. I sold that car for $14k. Another $6K or so, would've brought the power to supercar status.
If anybody can, it would be you. I think very few of us could, myself included. Still, $30K is a pretty good chunk and I suspect it's resale value would suffer (more so than the C5 anyway).
I'm not arguing just to argue. Honest. I would very much like to own an S30 pleasent enough to daily drive, without strangling it's general charecter, and not feeling like I've invested 1/2 my retirement or life. If there is a path to get there, I'm VERY interested.