Zaldog...low engine hours are not a bad thing in and of themselves if the engine has been well cared for. What you would be checking for in surveying a 30 year old engine with low hours is not much different than what you would do surveying any engine. On the plus side...there is little danger of normal wear and tear that you would see on an engine approaching rebuild time. Obviously all belts, hoses and other rubber parts (impeller, fuel lift pump) would need to be inspected for aging and cracking. The other danger with long spells beween start up is that the oil has settled and lost it's protective qualities in the cylinders during start up cycles...this could result in the scoring of cylinders and/or corrosion over time. Also oil that is not changed regularly can cause problems as it changes its' chemical composition. A good diesel mechanic should be used to check ANY engine during survey and will be able to spot if any of the above issues apply to the Bristol you are looking at. Hopefully, he will find that any issues are minor and you will have the benefit of an engine with many hours ahead of it before an overhaul is needed.
I wouldn't worry about the difference between 18 and 20HP and actually some technical sites give the rated horsepower as 22 so 20 is probably a good number! Solid workhorse engine that should do fine for you if it checks out initially well.