My 09 e350 wagon has a failed ECU. I’m told the new one needs to be programmed once installed. I assume Mercedes dealer or properly equipped shop?

 

If by "the new one" a re-virginized ECU (aka Motor Electronics or ME) it will need to be "married" to your car. Upon install, you use STAR/Xentry with "Developer's Kit" to enter the VIN into the blank Motor Electronics, confirm twice, VIN number is now programmed into the ME. You can now start the car. The VIN programmed into the ME is "hard-coded" into the memory after 20-30 key on/off cycles from what I have read. I did this Motor Electronic install one time for an acquaintance. I was slightly apprehensive about it but in fact it was quite easy. Of course, if you do screw it up your car will not start, much less run.

So...any good independent shop should be able to a do it for you.