Matron has had 20 years of MX5 NB's, and loves them to bits. The problem now is that they are all cornflakey underneath. But, like SL's there are a lot of really silly low mileage well-maintained second or third cars around that are looking for a new owner. The owners' club is huge, and the parts & expertise availability is phenomenal.
When the kids were small she had ten years of SAAB 9-3 convertibles which were very reliable. The only issue was a noise from the left hand side front passenger seat: "this is a bit boring, innit?" You'd easily get a 2007 9-3 LPT for that kind of money.
E46 320i would be a great choice. But hard to find anything with a 330i that's still in good order at your price, methinks
Why not a BMW Z3 ? They're not as bad as people think. They're just not as good as a Boxster.
SLK200 out of the question? Modern looking motor.