Ask for all, if any, service papers. Or try to find out when the following was done:
Spark plugs/wires/ignition rotor/distributor cap
water pump
coolant change
brakes
transmission flush
fuel/air/oil filters
power steering
check for oil leaks around valve cover and timing cover
check for leaks around transmission pan and bell housing
check the condition of the serpentine belt, should be no cracks
check the suspension, you can press down on the hood and trunk, the car should not sink down very far and there should be minimal bouncing, when u press down, the car should go down and come back up and that's it.
check the strut mounts, usually you can see visible signs of cracking from under the hood.
make sure all windows roll up and down with all window switches.

That's pretty much what I can think of off the top of my head, anyone else want to add?
Oh, and if any of the major components fail, then you can definitely subtract the cost from the asking price I think unless the owner has it fixed.

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1990 190E 3.0L