As already stated, you can just about get a mk2 for £3500. Infact mine will be going for around that in the next few months as I need to change to a saloon in the near future (oops).
A few thoughts on the differences between the MX5 versions.
- The mk1 is slightly lighter, has a more 80's interior, but is apparently slightly more fun in a raw sort of way
- The mk2 adds a slight weight increase, better torsional rigidity, glass rear screen that doesn't need unzipping and a better interior, hence why I went with the mk2.
- You'll be looking at either a later 1.6 or a 1.8. The 1.6 is fairly hopeless unless you just want to enjoy the top down feel. The 1.8 actually feels like a rear sports car, and with a decent cone filter makes a lovely sound when required, but doesn't give you a headache when you want to take it easy.
- AC and PAS are potential options. Personally I never saw the point in AC in a soft top, but that's just me. If you've got used to cars with PAS, don't buy one without. I tried an early mk1 1.6 with no PAS, and clearly have turned into a girly in the meantime, as I really found it much too much like hard work.
- If you go with a mk2 1.8, be aware that the steering wheel in the 1.8i is a nasty nasty plastic thing with an airbag. Mine had gone a bit soft when I bought it. The "S" has a fairly nice leather one. The problem with the plastic airbag version is that you really need to replace it with a similarly airbagged version. Brand new wheels are a LOT of money. Personally I was lucky enough to find someone selling a lovely Raid airbag/leather unit that makes even the "S" version wheel feel boring.
- The "S" also adds ABS, sports suspension and an LSD (along with 30kg), though typically they go for more money.
Hope that helps. There's a number of other owners on here if you require more advice.