I'm looking at a 2008 E63, but living in Minnesota I really want AWD. I know that the ML63 and R63 of the same time period have 4matic drivetrains, so it can't be that much of a stretch to convert an E63 to 4matic, right? The ML and R have different dimensions than the E, so I'm not sure a direct swap of front differential and other 4matic components would fit.

But could an E550 4matic donor do the job? There's less than 80lb-ft difference so I can't imagine the components couldn't handle the extra power. And then just flash an R63 ecu over to the E?

Buy an E550 biturbo 4matic or 2012 E63 biturbo 4matic. What you propose is likely very possible but incredibly cost prohibitive. Both in terms of flat financial cost but also the time and effort to do this. Donor parts may include anything up to and over an entire 4matic setup from one of these cars, substantial tcu and ecu alterations, sump modifications etc. Who would do it for you? How long would it take? The financial outlay will almost definitely end up well in excess of one of the cars mentioned previously once all is said and done. You would be winning to only be 18x over financial budget and 30x time budget.

I'm all for trying stuff like this! Unfortunately everything in this paragraph is true. If you can't afford to buy it, you probably can't afford to build it. Physically it may be as easy as you think if you knew all the right parts from all the right years. Electronically you may never get it right. If you decide to go for it, document everything well for others please. The knowledge is the biggest part of the battle. It's easy the second time.

I have 2 cars for now e550 4matic and my e55. Granted I'm in Southern California so rain/snow is hardly an issue. You can always pick up a 07-09 E550 4matic cheap as well as a second driver. As mentioned above if your looking for one vehicle 2012 E550 Biturbo is your best bet or of the same year E63. I can tell you my E550 4matic in the rain is just awesome, zero traction issues and just grips and goes. I'm glad it's staying in the family.