It probably depends how much DIY maintenance you're going to do and how much will need doing professionally.

I'd say a halfway house between a Beetle and an Austin-Healey would be a Triumph Vitesse convertible - the 2-litre mark 2, about 1968-71.

I was tempted by this when thinking about a fun car, but I decided that I hadn't enough mechanical skill or knowledge to fettle it, so went for a much newer model - a 2001 BMW Z3.

For a car in which to enjoy the journey a Rover P6 or early XJ6 would be great. If you want to get the top down, how about a Stag? All have great parts availability,to keep them running with ease, and there are specialists where you can buy one with almost no worries about rust or reliablity issues, at a price (but a price worth paying i think). Or perhaps a Mercedes SL (R107) for something a bit more substantial

But i wouldnt get too caught up in the tax exempt bit. even at its most expensive its just over £100 for 6 moths summer use. And the cut off date may move in the future. The exemption might even be taken away by some mean minded politicians, afterall we would be looking at 1989 F reg cars being exempt if the original plan had carried on.