Valve lash adjustment should be done around 600 hours. So if it hasn't been done, do it when you take ownership. Its not very expensive because the labor time is short. A few bolts are removed to take off the valve covers. Then about 30 seconds on each cylinder with a gap tool, then tighten valve covers back down. When I had it done the Cummins mechanic did not replace the valve cover gasket. I think my total cost for his time was in the $200 range. He also told me once you do the 600 hour lash adjustment, that it pretty much stays good from then on. Your mechanic may have a different opinion on that, so check that with them. Cooling system cleaning is a time thing for me. I do it every 2 years regardless of hours. Actually I have found the more I use the boat, the less crud builds up in cooling tubes in the two years. On each engine you have a gear oil cooler, an after cooler, a fuel cooler, and a heat exchanger to check and clean each time, and an impeller to change. Its pretty easy to do yourself if you want to, just takes the mindset to go through it one step at a time.