If I had 6K floating around me I'd seriously consider the E36 M3 but it does have its downers, one of them being part prices from the main dealer. Although its a solid and reliable car because it carries its infamous M badge its going to cost you a bomb when something goes wrong or needs fixing, my father is looking to order two new front shocks for his E34 M5 Touring and main dealer price is £840 + VAT a piece.
As for a Porsche well I've only ever been in one and it was an old 911 Targa and it was miserably dull and boring, I guess they've gotten quicker and improved over the years but I'm certain if you want something fun and pokey you'll need about 30K.
The Intergrale is a beautiful car too I find, I was close to buying one last summer but only the 1.6 HF Turbo. It was completely reconditioned and it had no rust but I was outbid on ebay

And they're not known for reliability and I guess the 2.0 turbo's are getting rare.
One car Id recommend although It hurts to do so is the MX5, my mum has the 1.8 Eunos roadster and its such a wicked car to drive. Its no 320bhp powerhouse but it will out drive a lot of cars on the road *cough* Toybota Supra *cough*
