Cayman S: Mid Engine RWD the way God intended! Though its hard to settle on one because they just kept getting better [and faster] as the years kept going... I had more fun in the brand new Cayman S than I did in the 911 Carrerra 4S but maybe just because I was less scared haha

I like your lists so far, I hope you find what you're looking for. I know people with most of the cars on your list and all are pretty happy with their choices. I agree that the Camaros and Mustangs are entirely too common for my taste, though a nicely modded SRT8 Challenger would be a very fun toy (this is my favorite Challenger build Rough Muscle: Jon Sibal's Dodge Challenger | Speedhunters), but they're a little old these days and might be worth waiting for the next iteration.

Keeping the Mercedes family I would probably go towards the SL, looks like you could probably get into a SL55 AMG.. but admittedly I haven't read up on them much as far as reliability or if they are worth it, just like the look.

I've been hanging out with a crowd of BRZ/FRS recently, and there are some really nice ones out there (the factory white on the FRS is beautiful) BUT they're essentially a N/A Crossfire when it comes to power to weight. The boxer 4 is an amazing little engine but is better suited for a curvey track than a straightaway...

The G8 GXP is a very cool car and was an M5 hunter when it came out.. you might be on to something there.. also, speaking of sedans, Buick is doing some interesting things these days under the radar. I was surprised to see Brembos peeking through the wheels of a brand new Buick..

And then there is BMW... my uncle had an e92 M3 and the thing was absolutely amazing.. loved it.. BUT for the money, I would personally get into a 335i and mod the bejesus out of it.. when the 335i came out there was a comparison between a Dinan 335i and factory M3.. the M3 still outperformed but for the price difference, even after all the mods, the 335i was the better car.. OR if you want to have the most fun, see if you can find a 1M or just a "regular" 135i with a tune and go fast parts