As long as you can still see to avoid the wooly bollards..
I've been having a similar look recently with a view to replacing the M3 with something slightly, er, less eye watering to repair.. there's so little out there though in the inexpensive price bracket. Obviously (to appease Kevin) I'm broadening the definition of "sports car" to include anything basically "nippy, quick or fast with better-than-average handling" and in my case "able to withstand a session on track without grenading"..
I came down to:
MX5 - except I probably don't fit (never tried but people who've owned them keep telling me I won't, so..)
S2000 - see above!
MR2 - been there, done that, though in your case I'd highly recommend an MR2 Roadster.. generally bulletproof, great handling, nicely balanced and require a little more skill than the MX5 as they are easier to make swap ends due to the short wheelbase and engine in the arse end
Boxster (early, small engine) - not that quick and expensive to repair, but there are plenty around
350Z - coupe is just in the <£5k range, convertible a little more. Great car, little heavy for track work but comfy, quick and decent handling (not as nimble as something lighter, obviously)
Monaro - just about in the price range, too big & heavy for track work without eating suspension though, I suspect (maybe 2woody can comment on that)
Porsche 944 - very old, not exactly a looker but a nicer interior than an early Boxster, no idea about reliability but I'm half tempted by
this oneSo far that's my list and it's making me wonder if I can find a way to keep the M3 instead as the M3 (reliability aside) does everything I could want of it; it handles track time well (as long as you remember to take a spare set of pads, ahem!) and eats miles with ease, comfortable, very quick, decent handling (hush, Kevin!).. the SMG gearbox is a pain in the arris around town as BMW couldn't program a smooth 3rd-2nd downshift if they tried, but on the open road it's a joy..