Several things here, first the rear window. Is it ONLY one rear window? If it is both, there is a switch on the console, drivers side between front and rear window switches to disable the rear windows, so that the kids can't play with them. If that's not the problem, see if the door or the console switch will work. If both switches fail, it may be the riser which will cost several hundred dollars to repair at the dealer.
Second thing. These engines had problems with valve stem seals, they had to be upgraded. It could be that this was corrected by '90, but I know it was a problem in '88. This is not a big deal, they can be replaced for several hundred dollars. The symptom of them being bad is excessive oil consumption.
It's very common to see these cars on the used market with greater than 100K miles, low mileage is a plus, but ensure that it has had all preventive maintenance performed.
Sounds great, enjoy,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in