The 420 wiring harness is made of the same flawed material as the 320 but there are two reasons why the 320 is more likely to fail - 1) as Paul mentioned the 320 wiring harness has to be disturbed to fix head gasket problems, and the 420 does not have head gasket problems, and 2) the 320 wiring harness goes all the way to the spark plugs, whereas the 420 harness goes only to the distributor. The 420 has convenential spark plug wires and replacing the spark plugs does require that the wiring harness be disturbed.

Both harnesses fail from both time and heat/usage. That said, a 10 year old harness with 40k is probably going to fail the next time it has to be disturbed. But if the car has been dealer serviced it may have already been replaced, even with the low miles. The dealer service records should show whether it has been replaced. Absent evidence that it has been replaced I would assume about $500 in parts and $200 to $300 in labor the next time it is disturbed for any reason.

The window lock problem I don't think would be expensive - sounds like a fuse or bad connection.