As far as I know they are pretty decent cars, we dont have a lot of 7 owners here so we can't help too much, but as long as the car was well maintained it shouldn't have any huge problems.
When you get it make sure you do all maintence items and get it up to par change all fluids etc. Also keep up with the maintence and you will have a happy experience, granted its a BMW so not always that cheap but it will save you money and headaches.