How many hours on the engines?
Not run for 2 years except for a weekly start and (?)idle, is likely to gum up the works a little.
A mechanical survey is a good idea, is there a genset to check too? With the sellers permission the survey mechanic might even fix the problem there and then.
Check if the raw water pumps have been replaced with American Diesel updated versions, the drive can fail unexpectedly in the originals, with an overheat unless you catch it quickly.
2005 Integrity 386 "Sojourn"