Nice one. As you say it's easy to go mad on any car to get it to it's optimum. On a budget you can have some fun with a mx5 on track.
Reference coilovers if your still looking and any after market or maintenance part you need (if you have heard or tried them already) try Andrew at Autolink. These guys get quite a few fresh imports in firm Japan for breaking. They often have decent secondhand coilovers for sensible secondhand prices.
You already know about nutz for a source of parts but if your on Facebook (and not heard of it) it's worth signing up to mx5 autojumble as there is lots of people selling various secondhand maintenance and performance parts on there. I got some light weight SSR wheels from there at a good price recently. There was also a chap doing some under bracing on there for too much money not so long ago too.
I be interested to see how you get on with your project and have to look on nutz and see if you have a project thread.
Thanks for the Facebook source. I'll get onto it.

I've already got a few parts from Autolink. They sorted me out with my gaskets and servicing things.
Not yet got a project thread on Nutz but I'll get round to it once I have some decent stuff to update.

Bob