Pretty good list there, The first thing to check as you mentioned will be the glow plugs as your stating issue most likely lies with them. As you have a 1980 model I believe you will still have the older loop style plug. You cannot test these by applying 12V directly to them. They are wired in series so if one of them dies those after it will not function. I believe the relay in your model is under the drivers side kick panel. If you plan on keeping the car for a while I highly recommend upgrading to the newer pencil style plugs, as these are wired in parallel and there fore much more reliable as a system. Also they heat up faster and glow hotter.
Also as maintenance history is not available you should change the two fuel filters as a baseline, as well as the oil filter, and air filter. A valve adjustment is most likely also in order.