So the M3 would be a daily driver and this would be a weekend car? One that is just for fun spirited drives and not 1/2 mile races or road racing?
If you wanted something blistering fast for relatively cheap, get a GTR.
It sounds like your budget would allow you to get into a 997 GT3.
The interior of Porsche vehicles is awe inspiring. I never liked the GTR's exterior. The car always looked very heavy and appears to be much larger than it actually is.
I would buy this if I was in your position.
There seems to be quite the price increase in these with the 991 GT3 coming only in PDK, so people with money who want to row their own gears, have been inflating these prices. Even if the market falls, I suspect that these cars would only lose 20-30% until the next appreciation curve. Therefore, depreciation and total cost of ownership shouldn't be as bad as the GTR and the C4S.
I never understood why people would purchase a $120k C4S. I'd rather spend a bit more and get a base model GT3. However, like one guy said at 6speed... GT3 wasn't comfortable enough for him, so I must respect that. 9k RPM in the 991 GT3 would be hypnotic enough to make me forget about lack of creature comforts.
Then again, I am magazine racing with an imaginary budget.