Opinions may vary, but 96/97 models are generally considered to be the best of the bunch.
I have a 96 500, owned 13 years, and a 98 500, bought last year. The 96 is physically the better car of the two, the interior being more robust and no signs of corrosion. That said, it was always garaged until last year, and I'm the 4th owner, and I'm the 7th owner of the 98, so not sure of its past, storage wise.
If a 500 is your choice, 96 on will give you a 5sp box and electronic ignition, thus preventing the constant niggles you get with the twin distributors you get with older models. The engine changed again in late 98 to a twin sparked engine, with slightly less horses, but a smoother drive with overall the same performance.
Spread over my ownership, I would budget approx £1500 p/a to keep on top of it, but I'm no mechanic, and I only use genuine parts (which includes new cats, a gearbox overhaul, and I was using the main dealership for the first few years). My mileage bracket is from 100k to 153K miles so far. I haven't started on or really used the other one yet, and it's going in for a service ++ on Monday.
They are great continental cruisers, with one of the best load capacities you'll find in a two seater convertible sports car. Panoramic roof is a very desirable option, and it should have a wind deflector too, so ask why it isn't present if not!!
Enjoy your search, both here, because they are well documented, and in the metal. Good service history and condition over anything else!
Edit: a bit of overlapping of Spike's post!