That is a nice budget so the market is your lobster
I would suggest you either buy a really nice MK1/Eunos 1.6 with the 115bhp engine and likely have a lot of budget left over or, depending what mileage you will be doing and how involved with the car you want to get, go for a MK3.5 2.0 at the other end of the scale.
Remind yourself what these are about by driving a few before purchase. a good time of year for a bargain but some nice days to enjoy the car.
If you’re used to a cx-5 (not a small vehicle) you might want to sacrifice the better driving dynamics of the mk1 (or indeed the mk2) for space, comfort and cruising ability of the (slightly bigger) mk3. Probably much easier to locate a nice mk3 than an earlier car too.
Mk1s are old cars now, with old car issues (rust, mechanical failures etc). Fine if you are into that whole classic car scene. But Mk4 cars on a 65 plate are now under £10,000. I think the Mk4 is a better car than the Mk3, and I don’t think there are an particular mechanical issues with the 1.5 model, which has more than adequate performance. The gearbox issues seem to be specific to the 2.0 version.
£4-5000 will get a fresh MX5 Mk1 from Japan, without rust worries, but possibly cosmetic issues. With that age of car, there are generally going to be issues of some kind to be sorted. So it depends how you envisage your retirement; tinkering with a car on a drive, or getting out to various places, top down.
Why an “older” model? If the price is right and you fit try a Mk.4 (ND), they drive brilliantly, give great mpg and being newer you should have less worries about the “rust” issue of earlier models.
Go to a proper specialist purveyor of Mx5s and get into a GOOD MK1, Mk2, and a later Mk3.
If the thought of a latter day Spit/ B / GT6 etc is in your DNA (Mk1’s superbly mechanically & simply engineered and actually finish journeys plus stick to the tarmac like superglue) , you’ll pick a spotless Mk1. BUT…it needs to be essentially rust free/proven resto’d or a rare escapee of the dreaded tin-worms.
For me, only the new ND’s get close, and I say this having a variety of them at a VIP day.
I own both a Mk1 & a Mk2.5.
The Mk2.5 is by a long shot a better car in various NVH & comfort ways but the Mk1 is my choice every time.
For me, it’s so much more driver & machine working together, and I’ve had these cars 15 years.
I’d give the Mk2s a body swerve(rotty botty risks…wrote the book, got the T shirt) and go straight to a proven Mk3.5 with good provenance.