With any used car you might find problems will arise.
If you are going between a 190E 2.6 and 300E, both should be pretty reliable.
You should be looking for one with full service records.
If it were me, I'd go with the 300E, because I like a slightly bigger car.
The 190e is a nice car, a bit small for me, but nice. They drive very solidly, and they handle good, and they get good gas mileage. There are also Tons of mods you can do to them, from body kits, to suspension tweaks. the 190e is small and easy to park when the spaces are tight.
My brother had a 1985 190e 2.3 and that car would cruise effortlessly at 120 mph. He also drove it about 350-400 miles on one tank of gas one time, with 5 quarts of oil missing from the car. And the car was fine. All it needed was some fuel, and to be refilled with 5 quarts of oil after that trip he took.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.