If a weekend, fun type car's what you're looking
for, I'd strongly recommend looking around for a MZ3 Coupe. Not sure I understand how much you're willing to spend ($18-$20k total, or that much down + some financing) so an '01-'02 model might be a little steep. But the '98 - '00 models are certainly within the budget. Different, less powerful engine in the earlier models, but they'll still get up and move very nicely. And they're all about the fun factor. I have an E46 M3 and while I'm happy with the car, it in no way comes near the fun factor of my old MZ3 Coupe ('01). I have no idea what I was thinking when I decided to sell it. I've kicked myself in the rear more than a couple of times over it.
A standard MZ3 convertible is nice too if you're into that kind of thing. And used Z4's are everywhere, and dirt cheap to boot. Comes down to what design suits you best. (plus, the Z3/Z4 community have lots of get-togethers, drives, etc if you might enjoy that kind of thing)