Your inexperiance does in no way reduce the consumers rights or expectaions.
As already advised, you need to get the car back to you and have it looked at.
As you've said, the customer is going to take you to court if you don't refund in full - why not spend a little money now, get the car recovered to an independant BMW specialist to have the problem diagnosed. You NEED to know what is actually wrong, and what has caused it to go wrong BEFORE offering anything. If he then takes you to court, you've got a report stating that whatever has gone wrong is because of 'XXXX'
Also, texters are chancers. Don't reply to texts, or they'll expect you to be up and immediatly replying to anything they say. Call, and speak on the phone.