If your covers are still useful I would repair the seats you have. It is rather simple work disassembling them. All those seat springs are going to be poor in used seats and the padding even more so.
A seat is very important in a car, probably more so that in front of the TV. Strange how people will pay $500 for a recliner but won't pay $300 for a proper auto seat. Its much more likely that you will spend six hours sitting in your car seat one day than watching TV in one seat that time (or anywhere else for that matter).
BTW if your covers aren't useful, buy one of the aftermarket covers but take the whole mess to a good independent upholsterer as they will have to be altered to fit as the MB ones will. Then you will have a comfortable, firm seat as should be in a Mercedes.
Ohh and by the way please fix your seats before spending money on wheels and radios.
Bosch Master, ASE Master, L1
33 years MB technician