Really?! 300,000KM isn't that bad for a diesel engine, especially a 1HZ. Unless it's been poorly maintained, I would believe that engine would have a lot of life left in it and worth rebuilding.
Are you purchasing a full donor rig, or just the engine/components? The only big issue I could see with only purchasing the engine is not having the proper electrical to go with it; unless of course you have a background in automotive electrical systems.
Do you have any photos of your LC? Where are you located?