Change all fluids (Oil), drain all old fuel from engine, lines and tank.
Look for a recommissioning procedure for your engine (it can vary from one engine to another). Diesels don't like to start and run on empty fuel lines, so good priming is required for start ups.
Prime the lube circuit too. Replace oil/fuel filters. IF you engine has a thermostat valve for the cooling replace that one too. They corrode and decay quite quickly when not in frequent use.
All of the above ... I also remove the glow plugs ( check the tips) and or injectors pour about 10-15ml of 2stroke into the cylinders and leave it overnight.
Before you spend a cent locate any sacrificial anodes ( these are made of a lesser metal than steel generally zinc) if there is nothing left on them you have problems......
Yeah the price of a dude is infinitesimal to your downside. But apart from all of the above id be worried about bacteria in the Diesel. Id get rid of it all. Rust in bores may be issue. Id gave a mechanic do what he feels necessary. Perhaps borescop cylinders, and in from exhaust manifold
