I have a 1991 190e 2.6 with 122k miles on it. Rust hasn't been an issue anywhere on the car. However, the head gasket is a common problem on that engine. Besides looking at the coolant, check the rear of the engine for oil. A common symptom is that it blows oil out the back of the head at the gasket. Another issue is the climate control. When these cars hit about this age, some of the vacuum diaphragms start to dry out and fail, as pointed out earlier in this thread. One common problem is that the center dash vents won't work properly. Mine, for example, always blows out cold air, even when the side vents are pumping out the heat (although one mechanic told me that the center vents always blow out cold air regardless of the heat setting). I'm gonna get that fixed soon.

Also, make sure you use the heavier weight oil in the engine though as recommended in the manual - minimally 10w40. No 30 weight oil for these cars!