Hi and welcome to the MX5 model dilemma!
The 5 makes a great “toy” car for fun and frolics on track and enjoyable top down touring - so here’s my opinion.
First 5 was a MK2, but brought in haste, without too much research, just bright red sports car lust after the kids flew the nest, so had a beautiful 1.8 Sport limited edition (just about every 5 is a limited edition of some sort ). I was tempted by the hard top, the VERY nice 17" wheels and standard body kit - little did I know as I was thrashing it around Brands Hatch with the worlds biggest grin on my face during my first ever track day - that it was rapidly disolving beneath me. A trip to the garage for a new clutch (started slipping just before I arrived at Brands!) uncovered the extent of the tin worm. Both chassis rails were almost invisible, the sills were held together by the body kit and the rear arches had more bubbles than an aero bar.
So, quickly moved that on to someone who wanted to restore it and started searching for the next one (once bitten by the 5 bug and I was hooked for life).
After traipsing all over the south of England I settled on my current MK2.5 1.8 Euphonic. I liked the seats, the wheels and the fact it’s got a LSD and the “big” brakes for the track.
Two more track days and the next one booked with the MX5 club at Oulton Park next month, I would recommend the MK2.5.
So, advice, drive a few, look hard and be prepared to grovel around on the floor (much to my long suffering wife’s dismay) to find the best one you can that has not started shedding too much weight. It’s a Mazda, therefore it will rust. The MK2 /2.5 are prone to chassis rail rust as they used a double box section for the chassis rails (Mk1’s only use a single box) so ANY moisture in between the untreated double box sections starts the self-destruct mechanism.
Mine’s not got a roll bar special suspension or anything else - just absolutely bog-standard (but with new discs, pads, oils and fluids and tyres along with a thorough service history). I have to admit I was nervous about track driving the first time, but it’s just soooooooooo much fun and a great way to understand how the five can embarrass cars more than 5 times what I paid for it.
Personally I don’t like the Mk1 (heresy I know and those pop-ups) but it’s too old for me and I wanted the VVT engine for more power.
I also don’t like the Mk3 / Mk3.5 as I think it looks too “frumpy” (cue the hate comments from the Mk3 owners - sorry, it’s just my own opinion and I’ve not driven one, just being honest).
I would love a MK4 - because they look like a fantastic car - but Mrs wife would string me up and I’ll have to wait for the mortgage to be paid off first
Whatever you choose - ask LOADS of questions here, look around, do your research and GO FOR IT. You will NOT regret it