You should post this on the Diesel Discussion, but I would recommend you drain/siphon all the old fuel out of the tank. Then use a commercially available commercial fungicide and fill with about five gallons of fresh diesel.

Of course you should check all fluids for proper level, be sure the brake pedal is firm, and fill the tires to proper air pressure. Then with a good battery crank it up.

You should do an oil/filter change ASAP, and also flush the brake system, and change the coolant using a 50/50 mix of Zerex G-05 antifreeze and distilled water.

Further driving and auditing of maintenance records should tell you what else needs to be done.

Duke