/diy/ - Do It Yourself

I am getting ready to buy a boat and will need to learn about maintaining a diesel and how to give a diesel the once over before buying. I have little experience with engine work and would call myself completely ignorant, I can do simple things like change oil or a belt and the like but that is about it. Any good resources? Google just keeps giving me wikihow tier results as any search I do shows my ignorance and google rightfully identifies me as clueless. Any general words of wisdom?

Also, pic related is the engine of a boat I am thinking of going to take a look at, it looks well maintained to my untrained eye, especially for a 40 year old engine. Does anything jump out from a visual inspection of the pic? All I can really tell is the mounts look good and show no signs of the engine moving, that hose in the bottom suggests something has leaked? What would turn the hose that color? Any idea what that hose would be?

Trying to decide if this boat is worth the trip to give it a look over, it is a decent ways away and will eat up an entire day, possibly two if I want to avoid spending most of a day on the road.