I can answer your question
or at least get a disagreement lol.
For rewarding trackwork that doesn't cost $$$$ you need something light, ideally not modified in the engine department and something you can afford to damage or at worst write off.
I've done trackdays in:
Road-spec standard Imprezas - too lardy and wallowey
*Highly* modified evo - too unreliable, too much $$$$ when things break or you have an off. However on big tracks you can pass GT3s...
E60 M5 - faaaar too heavy. Unless you have shares in michelin or can afford to stand at the side of the road giving every passer by a £20 note, forget it. Its a grand tourer, not a track car. I did a set of fronts in half a day.
Std M3 - OK, but like the road spec imprezas, too lardy and wallowey. Needs brakes, suspension basically needs turning into a CSL.
Atom - great experience but ultimately perhaps too dedicated. Not great in the rain :D
MX5 - FANTASTIC if you can get past being called a hairdresser and being overtaken on flats/hills. Superb car to learn RWD in and worth a packet of crisps.
306-GTi-6 - Great, but ultimately limited by FWD.
Megane R26.R - Greater, but ultimately limited by FWD. Possibly the fastest car you can use on track with a large degree of safety/ease.
CSL - er, hello ding ding we have a winner. Like an MX5 on steroids. Provided you could face seeing it damaged or written off and it sounds like you can't. So......
Had I not bought a GT2RS I would have bought a GT3RS. Enough of them around to replace it should you need to (or ask the insurers to!) plus don't modify it, don't make it "yours" and the next off the shelf one will be as good, no emotional attachment.
So there you have it. Sounds like you have a few quid so doubt you'll be interested in french clutter or hairdressers cars. I believe someone already made the recommendation about a GT3 Cup or similar - thats what I'd go for if a newish GT3RS is too much dollar.