sleeping stallion

ok i thinks thats what you call it. or rebuilding your engine.
what are the most common problmes why ppl rebuild there engines. is this something that u need to buy a whole new engine oryou can get away with replacing some parts.
i have 118k on my car and have a spare block im in the process of building dont no what to do that one yet. but as for the one in my car i spray 75 shot wet. im going to a 100 shot. since its high miles what should i be looking out for if i ever need to rebuild this one.