I purchased a 30 foot "Trawler" with a 6 Cyl 40 series Volvo of the same vintage (about 150 hp). The asking price was 40K--because of condition, eventually paid $25,000. This was actually a 32 foot boat, because of transom extension--had berths of 6, with an aft cabin, but was limited to about 12 knots.
I fixed the boat up, used it for a couple of years and sold it easily for $35,000. Granted it was not a C Dory, but it sold very easily as a running and cosmetically beautiful boat.
If I was in the market today for that type of boat, I would offer you $20,000 subject to a survey, and the engine running. (This does not necessarily mean in top condition). I think this combination of the diesel OD and the C Dory is a great combination.
Before I attempted to start that engine, I would first change out all fluids, Probably pull the injectors, and squirt some light oil into each cylinder. Then turn the engine over, with the "STOP" solenoid engaged--so the engine would not start. However even before attempting to do that I would at least rock the flywheel back and forth to be sure that all bearings were free. (actually doing this when the injectors were out, relieves any compression, and makes manual turning better).
To get the best price, I would have a Volvo mechanic go over the engine--may need new injectors. I believe that this engine has a turbo--and that should be serviced before running up RPM over 2400 or so.
Then go to the cosmetics, and I suspect that this boat will shine up nicely. If the colored stripes don't then you can paint with something like "perfection"--and make it look nice.
I agree that this is definitely set up for commercial trolling--and that should be the target market. There may be some C Dory owner or want-a be who is looking for this type of boat--and for them this will be a great boat.
I wouldn't do any thing with the electronics--they are very dated, and any buyer will factor that in, and want to buy his own electronics.
Good luck with the sale--and you have done a great job of presenting the photos and literature available!