Z4 fits the bill here I think. They don't have the same image as other BMWs, and roof motor aside, are reliable. Most of the suspension (and the powertrain) is essentially E46 330i so easy availability of consumables and advice if you like to do your own maintenance.
Make sure you get a 3.0i, and you should get some pretty nice pre-facelifts for your money. Leggy Si (the facelift with the N52 engine from the E90) are available from around £7k and £9k buys a really nice one. Slightly more for a Coupe except in the height of summer.
They just seem much better value than S2000s which are going in a silly direction in price, and, whilst Boxsters can be reliable enough, the possibility of the RMS or IMS bearing failing, destroying the engine, is a little scary.
Insurance is also really inexpensive despite the performance, especially in comparison with rivals.
350Z is also an option, though the convertible is a bit heavy really.