You're signing your own death warrant if you mix / match tires and do any sort of rallying in that car. You want equal grip at all four corners. Sometimes you can get away with swapping out new rear's or fronts, but only if they are the same make / model. Don't mix Tom, and I don't care if anyone disagrees, they are full of crap.
As far as ambient temp, that applies to Z rated tires more than what you'll need for that Benz. When you get into Z tires, things get complicated, and down right expensive.
I have always liked Michelin tires, you might want to hop on over to Tirerack.com and see what is available in your tire size. Nito makes a very good tire at generally, a lower price point than Michelin. I'm beginning to think Goodyear has forgotten how to make a good tire.
Yes, unfortunately, I have to go with run flats when I burn through the Nito's on the car now. Shouldn't be long at the rate I'm going.... 50 miles is better than nothing. It might just get me into cell phone range in nothing else. Plugging a tire on this car will be extremely challenging if it's on the car. There's no way to actually lift it to change a tire without a low profile floor jack. I have four inches of clearance in the front too. I do have a can of fix a flat in the car, but sure hope I never have to use it. The stuff is about impossible to get off the rim/tire and sensers.