£3k will get you one of 3 generations of MX5:
Mk1 “NA” (pop up lamps); Its possible to find one for £3k or less. It will likely have had welding to the rear sills done, which might be good for a few years, or might need redoing at a cost of about £700 per side. Mechanically robust and simple. UK spec cars might have factory immobilisers which give problems 25 years on. No real difference now on rustiness between grey imports and UK spec. Generally rust confined to rear sills and rear arches; treat these as one and the sam (ie. if rusty arches, factor in an assumption that the rear sills will also need repair, and vice versa). Very rusty cars might also have rusting on the front wings, particularly front corner. These cars are also more likely to have rusting elsewhee, to the floor pans etc. Probably no economic to repair. These cars are in an ascendancy with regards to value.
Mk 2; “NB”. 2 main flavours, 1998-2000 NB (or NB1), and a face lifted car “Mk2.5”, “NB2”, “NB-FL”. These also rust in the rear arch/sill area, but also commonly, on the front chassis leg. Essentially, repair of the commonly rusted areas will be double the cost of repairing the common areas of the Mk1. The front rust damage is harder to spot without the car on a lift. Repairs might be quite variable in quality. While external rear panel damage is quite superficial in terms of structural importance (its the inner sill that is more important, and, except in exceptionally rusty examples, the inner sill often survives well), a rusty front chassis leg is ALWAYS structural. One test for the rear sill (and for the Mk1 as well), is to jack up the car using the tyre jack and examine that jacking point (sometimes, in repairs, the lip is faked and has no strength. If the inner sill is rotted, the lip will distort under the weight of the car)
Mk2s are also mechanically simple and robust; some early Mk2s (and very late Mk1s) can be really oil burners, due to sticking rings. Additionally, the cars do become more mechanically complex, and not necessarily more reliable adter all this time.
With a £3k budget, it might be possible, through a private sale, to find a decent car, morel likely a less popular 1.6 mdel. The trade will be aware of all these issues, and I would not expect a £3k car on a forecourt not to have rust issues and/or a fair bit of filler lurking.
Mk3 “NC1”; £3k will likely get an early Mk3, with 2.0 or 1.8. If there is rust, it seems to be that it will be everywhere, rather than confited to a few spots. More complex, and the MZR engine is less robust, being less forgiving of oil starvation (oil surge under acceleration). Replacement 1.8 engines are hard to find.
All 3 will hav a diferent character. Mk1s will feel very old. A Mk2/2.5/3 at £3k will still be depreciating towards banger territory. A £3k Mk1, even one needing repairs, is in the ascendity.
Which depends on your needs. The Mk3 will be the most modern, wrt safety etc… The Mk1 willfeel the oldest, and likely need to most regular tinkering. If you are looking for some occasional use type of car that won’t need loads of mechanical work, I would look for the best £2000-2500 lowest mileage, lowest number of owners base model 1.6. It also seems that grey imported Mk2s were more likely to be 1.6 models (and potentially, less rusty).