If well maintained, those motors can last a long time. A few suggestions: See if you can find out when and what routine services were done. If you have no idea, it's best to replace the water pump (and housing;full kit) and the thermostats. While the tstats are out, take a look inside the housing for buildup. If none, great, if a little, it's OK. If heavy, you will need to use a salt disloving flush like SaltAway and applicator attachment. Replace the gaskets when you change the stats.Pull and inspect the plugs. If needed replace them. Those plugs are relatively inexpensive.Pull the prop and inspect the shaft for fishing line or oil leaks. Drain, inspect the lower unit oil. The lower unit holds a tad over a quart of oil. Try to collect and measure the oil you drained out. You won't get it all but if there is less than 3/4 of a quart, you need to find out why. Check for mikly white color or metal particles. If you have access to a tester, this is the time to do a pressure/vacuum test. It's nice to know but if everything checks out, the test isn't critical. The lower plug has a magnetic stud on it and usually will have a bit of metal filings on it . That is normal. Milky oil is not normal and indicates water in the oil. When you refill, use new gaskets. They are also pretty inexpensive and in general should not be reused.
When you gas up, use a fuel additive to reduce carbon buildup. RingFree is Yamaha's brand but take a deep breath before you look at the price
There are other similar products that cost less. Note that these products are not fuel stabilizers. I don't use stabilizers during the boating season, just for winter lay-up.
Flush the motor after every outing with fresh water.
Unless you have a real good good reason to do so, do not pull the O2 sensor or open the VST tank. As is the case with boats, I am sure things will pop up. Address then as they happens