First off, welcome to BMW and the forum. It's true Mercedes' tranny's shift much more harshly than a BMW. However, if your transmission is slipping, that's not the way it's supposed to work. I'd suggest taking the car to a independent BMW tech. I'm pretty sure '91's dont have a dipstick as the transmissions are sealed. If it is low, a good tech should be able to top it off.
You can get a replacement owners manual from the dealer for around $16. If you plan on doing any maintainance yourself, you should get a Bentley Repair manual. They run $35-40 and will pay for itself the first time you use it. That along with this forum will be invaluable for maintaining your car.