I'm reaffirming my position: go with the Merc. Even if it doesn't have much history, just make sure you test every button and function, bring a rag and look at the fluid levels, poke and prod things, walk around the car before and after the test drive and check for rust and frame damage. That's my set of guidlines.
On the 190e's, watch for rust under the wheel arches, in the trunk under the carpets, check the weatherstripping, check the aircon, electric windows and the defrost functions. Check to see that the dash is well illuminated when the lights are and that they all come on, finally, make sure the steering is tight and has no play. If you need some specific questions answered relating to costs or parts or anything, feel free to PM me.