First and foremost, check the coolant level -- low coolant will cause rapid temp variations. You are in danger of ruining the engine by allowing it to overheat!
If the coolant level is OK, there are several other things to check, but DON'T drive it with the temp guage going up into the red zone unless you are CERTAIN it isn't really overheating. Otherwise, the problem will solve itself, the engine will lock up. Check the wire to the temp sensor, if it is loose or worn, it can short, sending the temp guage up. Sensor can be bad, too.
There is a manual operating nut in the trunk for the sunroof. It is behind the trunk lining, on the drivers side, I think. 14 mm deep socket, should crank relatively easily.
However, you need to get it lubricated before you get too excited, it will only stick again. You also need to check the condition of the lifting angles it if tilts up -- the earlier ones break easily and will jam the sunroof.
Brakes that whistle or squeal when not applied are usually dragging. There are several causes, the most usual is that someone didn't replace rotors when needed, allowing the pads to jam on the antirattle spring. Left too long, this overheats the calipers and necessitates replacement of all the rubber parts. Shouldn't be a problem for you, you don't have enough milage to require rotors yet, unless abused.
Squealing when applied usually means the wrong pads are in there -- MB pads wear pretty quickly, and most aftermarket pads are too hard. Makes them last forever, but they don't work as well and screech. This can be solved by replacing with MB pads.
If you don't know the maintenance history of the car, ask for a "major" service -- any MB shop will know what this means. If your shop isn't familiar with MBs, find another one (best choice) or educate them -- if it were me and I had the money, I'd have the coolant, brake fluid, transmission fluid, and oil changed. Probably the rear end lube, too. Inspect brakes, repack front wheel bearings, and do an inspection. Lubricate throttle linkage, check hoses, etc.
The 190 is a nice little car, but like all MBs, must be maintained. Neglect them and they will fall apart.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!