[jfirwin]

Great DIY! Thank you.
I just replaced the water pump and thermostat on my '08 because of a check engine light and a stop at Advance for a free code read.
The code actually said that there was a difference between what the temperature was and what was expected, and the thermostat needed to be replaced.
Code P0128, btw.
I ordered the Cooling System Refresh Kit from ECS tuning, part ES#2777070 because I wanted to replace the hoses while I was replacing the thermostat.

Couple of things that I learned:
1. You don't need to replace the expansion tank and cap, I read where previous generations of M3 had expansion tank cracking issues, but my 80k mile e90 looked great
2. ECS "recommends" a coolant evacuation tool (part 2712734) for $95. I wasn't impressed. The grommets supplied in the kit didn't create a solid seal with the expansion tank, I wound up getting a gallon of 50/50 into the system, running the engine with the heater on high until the car was hot, then letting it cool down and adding the second gallon by hand.
3. When replacing the hoses, remember that they are "keyed", do not twist to remove them from the radiator! Let me tell you, they were a pita to remove from the radiator. Especially because the radiator neck material appears to be the same material the hose connections are made of. You could as easily break the radiator connections as the hose connections.
3. You will need a 1/4 hose clamp to replace the one you will have to break off of the overflow tube on the old radiator upper hose.

edit - ECS sent me the wrong belts, the correct one for my pre May 2008 M3 are 6PK1460 and 6PK1067