Is it easy to replace a window on a BMW 323i yourself?
First of all, if you have decided to work on your car yourself, you need to buy this book: BMW 3 Series (E46) Service Manual: 1999-2005 A lot of DIY repairs and maintenance can be done by simply consulting this manual. Regarding your specific problem, power window repair is indeed something that can be done by a reasonable person who knows how to handle a screwdriver and a wrench.
Window work is especially difficult, because it involves a lot of work to take the door apart and so on. This is part of the reason that dealers think they have a right to charge a couple hundred on just labor to replace a window or even just put it back on it’s track. I wouldn’t recommend attempting it yourself. If you really can’t afford to get it done by a dealer, the only alternative would be getting a friend who has experience with it.