Diesel fuel can certainly go bad by growing algae.... even cars being used can do this... I think it would be worth syphoning out the old diesel and putting in fresh... but not much ... like three gallons plus a can of Diesel Purge.... and replace any filters you can reach easily.... Any filters you replace put Diesel Purge into them from another can.....This way you are doing a mini Diesel Purge injector cleaning right off the bat.
I would also change out the oil and filter if the rest of the car looks good enough that you think you want it.
Then if it starts and you buy it replace all belts and hoses as soon as you get it home. And flush the brake fluid completely before driving it. Is it auto or manual ... be sure to check the trans fluid level before driving it... if auto consider changing the fluid and filter and adding Trans-x.
What reason does the owner give for the car not being driven for that many years ?