You might wanna check out things by yourself, if you really know anything about cars. Even if you don't you can always check for rust by feeling around the skirting inside (also look for visuals on body). Open up the hood and look at stuff (you can see lotta things in there. When you take it for test ride, you can feel the car and might be able to tell what's good/bad about it. Also when you turn on the ignition, it will light up the 'INSPECTION' bar for few seconds. It goes from green to yellow to red. If it is on red, then it means it's due for INSPEC I or II. INSPEC I at dealer would be around $350-400 and INSPEC II would be around $650-750, if everything is good inside.
The best thing to do is to get 'buyer's check' done at some good place. Also get it done at BMW dealer. You would also get an idea on repairs $$ as well as things which one of them might miss.
Hope this helps (I assume that you would check the car records at autocheck or somewhere before buying).
Good Luck shopping.
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