I had an M140i (with Bilstein and Mpower LSD) from new as my daily alongside an MX5 as my play thing. Used the BM on trackdays, but if the weather was good preferred to drive the MX5 on the road (unless covering big distances).
As MadMalc points out above, the thing with the MX5 is the massive headroom. Gives you almost bike like feeling of freedom. If that doesn’t matter much to you (and you wont know until you try it for a couple of weeks) then there may be better options for you.
Personally, I caught the cabriolet bug from a BMW E93, which I traded for my M140, then found I missed the open air thing and bought an old rust box MX5 with a view to building a kit car. Loved it so much I bought a better MX5 which I still have, and eventually sold the M140 as it wasn’t getting used when I wanted to drive for pleasure (bought an old SUV as a winter hack/box shifter).
Compared to the E93, the MX5 is much nimbler and more fun to drive, easier to park and still get out of (doors on the BM were the length of a football field) but not as spacious or luxurious.
I explored mods, specifically forced induction, but once I owned the car for a while, the charm in them is not (for me anyway) in more and more power. So I spent money on suspension, hifi, and petrol and just run the standard 160hp, which is no rocket ship, but no slouch.
But I am aware there are a lot of us 50+ blokes driving these things, and it may be that we have “been there and done that” when we were young and maybe that is why the adrenalin side of firebreathing horsepower machines does not appeal to us as much as it once did. The grin factor comes from driving and being at one with the world (sorry, went a bit Zen there…) as opposed to the (admittedly huge) grin of letting 400hp break loose the rear-end as you launch from 30 to 100 in four seconds spitting the world out behind you. It is a very different feeling.
So, if I were you, I’d keep your £15k and buy an NC 2 litre for around £4 or 5k and run it for a couple of months as a second car - then decide what to do. You then have the option of selling it for what you paid for it and moving on, upgrading it (some have even fitted v8s) or buying a newer shinier one. If you jump straight into buying and modding you will likely have a bit of a money pit that you will never get your money back from - and if you dont end up enjoying driving it then you may feel a little bit burnt by the experience.