Just a clarification, what's described above isn't a runaway diesel, it's just a vacuum shutoff failure, not a big deal. On the other hand, a runaway diesel is an unusual event where the engine goes to maximum speed and self-destructs in a matter of seconds/minutes.

Regarding the question, the 83 300TD is a W123. You didn't tell us how many miles are on the car. There are a few things you should pay attention to. The automatic transmissions can start acting up around 250-300K miles, so look for "flaring" between shifts. The automatic climate control and AC systems can be flaky and fairly expensive to fix. The cruise control systems can also be fairly expensive to fix. The vacuum door looks can act up, but are fairly easy to fix. Look for rust, structural rust can be fatal to these cars. Definitely get a pre-purchase inspection if you find a promising car. Good luck.