Blowby alone could work as a driver, the only problem is when the compression gets too low for the engine to start. Compression is what you're looking for.

The good news with a diesel engine is that as long as you have enough compression for the engine to start and can carry enough oil to get where you're going, you can continue to drive the car. A gas engine burns valves, fouls spark plugs and other ills. The bad news is that if it does get to the point of needing a rebuild, it is typically much more expensive to drive a diesel.

I would suggest that you do a compression check on this engine. If the compression is 300PSI or better on all cylinders, the blow by itself will not pose a problem that will prevent the car from being drivable. Typically excessive blowby will go along with low compression. But it could be that excessive oil consumption will not prevent the car from starting and running.

You should do two things, see that the compression is up and that the car starts when cold. I have heard of people using the block heater to start the car so they can sell it to an unsuspecting buyer. Don't get caught by that one. Before you start the cold engine, feel of the engine to ensure that it is indeed cold.

If it starts easily when dead cold, and I mean overnight cold, and it has good compression, you MIGHT get by with this for your purpose as long as your son understands the necessity to constantly check the oil and keep it full.

If you do buy a serious oil burner, don't believe the old wives tale that the oil changes itself. Contaminants are worse in an oil burning engine, and need oil changes maybe even more frequently than a healthy engine.

All that said, I really don't recommend that you go down this road. You may very likely spend more on this car in the long run than if you had spent more money up front for a solid car.

Best of luck and Merry Christmas,