I suppose the best place to start would be to learn how to conduct the service routines on the car, oil changes, brake checks, coolant flushes, spark plug replacement, air filter and oil filters.
As for the starting, I guess it depends on what the car has fitted, 1990? Well (and these are guesses and open to correction) does it have automatic choke? I had a 1990 car with manual, and a lot of people have now forgotten how to use them, automatic chokes where prone to running the cars very rich for too long and this caused all kinds of problems, perhaps your starting issues are something to do with that. I would not expect a 1990 to need you to put your foot on the throttle to start it, should be key turn and choke setting at the most.
Yeah, they are right.. don't use water within your cooling system... If the radiator was half full I might be a bit concerned, perhaps it's using water which is a potential head gassket problem.. BUT.. I would recommend a complete flush of the cooling system, and replacement with a properly mixed BMW coolant fluid. It is not unusual for these cars to use a little coolant now and then, nor is it really anything to worry about.. All my BMW cars have done this (E36 models), and I simply check the levels once every month or month and a half... about every time I check my washer fluid, I always check my oil too.
I think on a car like that.. I would start by making a list of things you think might need doing, and then working through it, a full service and coolant flush would be my starting point. On a car of that age (assuming the mileage is also high) you might expect the following:
Suspension bushes Suspension mounts Suspension Shocks Track rods Ball joints Clutch Wheel bearings Various body work - particularly wheel arches, boot, and where the bumpers meet the main body.. stay on top of this, she'll thank you for it later.
Apart from that. .I think these cars are fairly robust.. and she should give you many years happy motoring..
Hope all that helps.. Welcome to the BMW club!
Craig