I would just open the coolant cap to let some of the pressure out. You will loose just a little bit of coolant. Be carefull with with four screws, it is easy to drop one. Be sure to carefully examine the old one to see exactly how the new one goes in and the washer placement.

The monovalve in your car is against the firewall, right next to the battery. I did NOT take the battery out because I did not want to mess the the radio codes, but there would be more room if it were removed. There is not alot of room back there. The part is available from any mail order foreign car place.
(Mercedes-benz-parts-direct.com or alloemautoparts.com or many others.)

There is just one power plug-in on the top. Sometimes pushing the little side clamps can be difficult in such tight quarters.

There is a temp sensor on that car in the sunroof control panel. It has a tiny, square grill opening about a half inch square.

Good luck

Good Luck