Used 911, would need to be air cooled to be extra cool, (non-turbo 3.2 Liter unless you like paying a lot on maintenance). You can find Cayman for around $20k also to be had are Lotus Elise (Honda or Toyota motors in some). Another not on your list BMW M5 (used). The Z4 is a really fun car, but when I have driven them they have limited head room on entry and exit, and the blind spots are large. The rear window is very direct so you need to guess when changing lanes that nobody is there. So go test drive one. I like the 350 and 370. The 370 is nice and you can get what all you want on it. 350 are just now getting really abused by second and third owners so you can still pick one up unmolested.
I am sure you are aware that your Audi will need a new timing belt in about 5,000 miles, and at only $1,500 approximate it is a pain. Then you will need new brakes soon too, maybe calipers ($$$$$). If it were me, I’d also be picking out the new replacement.