I think condition and servicing history are the most important factors when looking at an older car. I would forgo a newer W124 over one that was a couple years older if the older one was in better condition and better maintained. I would drive both M103 and M104 cars and see which one you like more. I could see the more powerful M104 being better suited to the larger and heaving wagon than the M103. Check the car over carefully and make sure to get it inspected before purchasing. There are a lot of electrics to check over, so make sure that all those switches and accessories are working as they should. On the wagons the rear tailgate self closes itself in so you do not need to slam it shut, so make sure it is working.