The 500s aren't that bad. They rarely get the timing chain issues the 350s see, and they are solidly built cars. The 7 speed may be the weak point in the car though, when they came out Mercedes wanted trans services every 40k miles, and most people ignored that. Theres still 300+ hp going through that in a heavy SUV, so that may be worth paying attention to on the test drive. We replace the SAMs pretty often, these are the Signal Acquisition Modules, pretty much the fuse boxes, and they can fail with some odd symptoms. Trunk will auto close but not latch, or trunk will attempt to open without unlatching. Brakes aren't terrible for the price of the vehicle, usually at my shop its around 500 for front pads rotors sensor installed, rears around 425. Unless you have the larger brakes, then its in the 700s in the front. At a discount. Oil leaks at oil separator and cam plugs, thermostats fail regularly, we did 2 this week on an e350 and a c300. Roughly 550 with a belt for the thermostat.
