My .02, based on owning quite a few sports cars that people would regard as pretty serious cars- most modern 'sports cars' are too big, too heavy and are probably more about comfort and horsepower/torque bragging rights, and the odd stoplight drag race, or autobahn style comfort cruise, than anything that really involves the driver, rather than cosseting him. Some don't even offer a real manual transmission. (I know you can snap a shift faster than Michael Schumacher using paddles, and concentrate on your line, without the awkward choreography of foot clutch and stick shift, but in the real world, I'm not interested in milliseconds, I'm interested in an engaging drive). It's almost like the difference between the objectivists and the subjectivists over what makes an audio system 'good.'
I'm excluding really old cars, and most vintage British stuff, because I assume you actually want to drive the thing, rather than visit it at your mechanic's shop. i'm also excluding almost all the vintage stuff that is really worth owning, because most of it is far too expensive today, to own or maintain. (And fwiw, I'd still rather have a short wheel base 250 GT than the 250 GTO that just sold for Van Gogh painting prices).
So, based on my experience in owning a bunch of cars in the Ferrari (six of 'em), Lambo (6 liter Diablo), high-end MB (not really a sports car but a 69 6.3 litre), Porsche (GT2 club sport), etc. category, I'm gonna place my vote with an older Porsche. 911S would be the best but we'd be bumping against the budget. Air-cooled, preferably. With some small amount of work on the suspension, no heavy duty tricks on the motor, no turbos, no blingy wheels. 993 is probably the most up-to-date version of the cars I'm describing, but even it is a little on the heavy side. You could go back all the way to the late 60's with price variations. Leaving aside the few uber-collectable or race oriented iterations, e.g. 73 RS or 993 GT2, most of these cars, bought right, can be 1/2 the proposed 100k budget or less. Fun as hell to drive, actually race-able if you are so inclined and can hold value. Colin Chapman was right about making it light. There is a reason why a lot of the modern 'sports cars' need 5 or 6 hundred horse power. Do you know how much all that electronic junk and comfort and convenience weighs?
Porsche has been winning serious car races for a long time for a reason. The preferred car on the Targa Florio and on the old Nurburgring is probably a Porsche. Fast, good on the twisties and pretty reliable, overall. The newest ones are wonderful, but we're over budget, and if you are going there, spend the money and just buy the Carrera GT, you know, the one made back in the mid-2000's that killed a number of enthusiasts on the street and track. That's the best car I've ever driven, and it was scary fast, you just didn't realize how fast, which was why it was a killer.
Ya just can't drive fast on American roads anymore. (OK, you may find exceptions here and there, but how many of you regularly drive in places like Montana- and is it a sporting experience to drive at high speeds over long stretches? Not that interesting, really).
So, you take your fun at lower speeds. Seeing 'how fast' you can go gets dull pretty quickly anyway. My last 'exotic' was a 1928 speedster from an obscure British company- quite a handful at 55 mph.
I'd also consider a Morgan 3 wheeler, for that bugs in the teeth sorta terrifying at any speed experience. (I haven't test driven one yet, but plan to soon). I have more respect for a tuned Miata than I do for a 550 hp car that weighs two tons. My true love is the really old good stuff at this point, but that's not practical and certainly not cheap. A sports car should be fun to drive, and challenging to you as a driver.
That, to me, like the 'soul' or 'musicality' of a hi-fi system, is what a sports car is all about.
