I am not sure that the situation is all that dire. Diesel is a good lubricant/corrosion preventative and I have heard of many pieces of construction equipment that sat for this length of time and started up reasonably easily.
But sitting near saltwater is a problem and salt air entering through the exhaust will have a path to a cylinder or two.
So bar the engine over to find out. If it won't turn over with reasonable force on a 1//2" breakover bar then walk away unless you are prepared for lots of work or an engine replacement.
Sometimes you can break them loose with lots of soaking with WD40, Marvel Mystery Oil, etc. and patience. I had a friend whose boat sat for 18 months after IRMA on its side and when finally righted the engine was stuck due to rain water seeping in. Patience and a week of soaking finally broke it loose and it now runs fine.
If it does bar over then you are almost there. Change the oil, filters, maybe polish the fuel, change the filter, change the impeller. Check the rack on the injector pump. Sometimes they will stick and need to be rebuilt otherwise it can run away.
Then pull the stop lever(edit since it is an energize to start, disconnect the fuel solenoid wire), crank for ten seconds until you see oil pressure and close the stop lever (reconnect the wire) and go for it. If it doesn't start in another 5 seconds stop cranking and close the raw water intake so you don't flood the engine and investigate.
Fuel, air and compression starts diesels. All have to be present to run so check them all out.
David