I can almost write a book on this based on the materials I have collected for the last 2 weeks of shopping. Nothing in my shopping cart yet..
Seen both lemon and jewel but the problem is if its a jewel, you need to confirm there and then. One day of delay to decide, only lemon are left behind, the jewel will be snatched faster than u can imagine
The are so many angle to look at but since u only asked a generic "what kind of condition", you will get my generic answer or rather opinions.
1. Go for the newest manufacture year.
2. Go for the lowest mileage.
3. Insist on service record if you know how to read them because lots of story can be told about both the owner and the car by looking at service records.
If you have seen CSI enough, you should be able to tell how a car is driven/treated, guess its mileage (odo can be tempered) and to tell if its accident free or not. There's just too many things you can look at. For example, if u see the brake pedal rubber pad has been stepped until can see the metal and odo only show 25k km. You know someone is not being honest. Oily or missing tools in the tools compartment is suggesting a road side break down has occured or something, and many more...
An experience foreman will even tell if the car is a potong car by looking missing clips and OEM fastener on the chassis which are not suppose to be meddle with unless there's structure alteration. Obviously, I have not work with BMW enough to identified this.
My problem is I get attracted by bodykit and big rims.. damn..
Luckily my wife is always around to pull my hand brake.. "Dear! are u sure? Look at the year / mileage.."
Another way is to be patient and get your trusted foreman to recommend because if the owner maintain the car at the foreman's workshop. He can definitely tell the lemon from the jewel. That will be the best bet I can think off.