Looking at 325i with sunroof issues

My son wants another BMW and there is a nice 2002 325i about 50 miles from here with about 111K on it and other than a minor dent the only issue is the sun roof popped out on the current owner and he simply put it back and unplugged it. I have done some research on these and looks like it could be a bit tricky to fix for someone not used to working on them....My question is how much would a repair shop charge ( ballpark) to fix one of these things and how hard are they for the average joe to fix? I realize not knowing 100% what it is does not help but looks like most of the BMW's I researched all have the same parts wear out.... The guy wants 4500 for the car, trying to make a realistic offer on it if possible.