Sounds like the radio code. I assume you saw this in the LCD of the radio. It means the power to the radio has been interrupted such as by disconnecting the car's battery for service or replacement. It is part of the car's anti-theft system. A small credit card sized card with a numeric code may be with the owner's manual set (or sometimes people write the code in the back of the owners manual). If not, a dealer can look up the code. You will need to show id (DL) and the registration to prove you are the owner. No charge. If the dealer code doesn't work, the radio may have been replaced by an previous owner. In that case you may have to contact the radio manufacturer to get the code. This is, of course, designed to discourage radio theft since w/o the code the radio won't work. Directions for using the radio code are in the radio manual which comes with the owner's manual set. If not, sites like this one can assist you.

The dealers were being straight with you. It'll be a lot easier for you to get once your the owner.

You didn't ask but may I suggest that when you narrow your search down that you pay for a pre-purchase inspection. These cars are pretty complicated and stuffed with interconnected electronics. Good news is that there are a lot of E320 wagons out there so you don't have to marry the first pretty one you meet.