Elmer:
Yes 6LPs had the notorious valve defect, but Yanmar (was forced to maybe) replaced them with better metallurgy and no further problems have been reported. You may have to pay to replace them if the engine you buy hasn't had it done, but that is rare. The 6LP does have a belt driven camshaft that needs to be replaced periodically. But with the new metallurgy valves and attention to the belt replacement schedule, the 6LP is a solid engine.
If you are a reasonably handy DIYer you can do the complete exchanger service for the cost of a few O-rings. Not cheap at maybe $100 but no where near the cost of a mechanic. And FWIW I did a complete aftercooler job, rodded the main heat exchanger to open the tubes and acid flushed the whole system in about 20 hours which should cost $2-3,000 at today's mechanic rates.
David