Similar advice to what the others have said but whether you go for a 240, 740 or 940, get one with fuel injection, preferably L-Jetronic, LH-Jetronic or Motronic as they will be more fuel efficient and generally less troublesome than carbs or K-Jetronic although most K-Jets are very reliable too.
No fancy computers are needed to sort out problems on them either!
Also don't be tempted by the so called "economy versions" with the smaller engines, they aren't suited to the heavy bodyshell and will reward you with poor performance, heavy fuel consumption and increased/premature engine wear.
Get the 2.3 in whichever model you go for and you get reasonable performance (lively if it's the high pressure turbo but a youngster may struggle to insure a turbo) and economy. The 7/9xx ranges are both just starting to come up in value as well (unlike the 24x which started climibing some time back) so there's a good chance you could buy a "fixer-upper" for a few hundred quid, sort out its little foibles all the while your lad will be learning and end up with a car that is worth more than it owes you and is still going up.
