Head gasket and wiring harness issues will only apply to 300E (or E320) models from 93-95 equipped with the M104 6 cylinder engine. Prior model years (for the four doors only) were not equipped with this engine, and the 190 never came with it either. Unless you were looking at a 93-95 E class you don't have to worry about either of those things.

The general consensus on either of these models is that they last a long, long time when properly cared for. Hundreds of thousands of miles. As far as maintenance, it would be helpful to know what you drive right now. If you're driving a Honda or a Chevy or something like that, you're going to be a little surprised at how much more regular maintenance comes with owning an MB. If you're driving an Audi or a BMW you won't be.

I'm going to assume you're looking at cars that are 12 years old plus, because you've specifically mentioned a 190 and those haven't been made since '92, I think . 300E's are of similar vintage, with the newest one you'll find being a '95. With *any* 12 year old plus car, you better prepare to invest money in a lot of the things that are ready to go. Suspension components (MB uses a lot of rubber that wears out) and cooling systems (radiators, water pumps, etc) come right to mind. This is not just for MB, its for any car that old. Things just start to break, and that's just gonna cost you money.