Hey, i got an 83 300D with two Xtant A1240's in the trunk so i feel your pain. First off, these style cars have the gas tank in between the trunk and the back seat, so you have to improvise. If the car is equipped with a First Aid kit relocate it and put a carpeted baffle made from 1/2 MDF board in it's place, make sure to drill large holes in the board,to allow air to move freely. there is also a sheet metal wall in the back of the trunk, replace that with thinner MDF as well. Ned's right about insulation, when I have a customer with a Mercedes come it I recommed they use Dynamat (or a generic brand) on the rear of the trunk, rear quarter panels, spare well and trunk lid to seal the trunk as much as possible (besides no one likes hearing rattles.)

Any deck should fit, although i've encountered problems with later (85-90) MB's requiring some dash cutting. This should only require a bit of shaving with a utility knife however, any more and your radio isn't a normal DIN width.