The fact that the 3 cylinders are all identical pressure would indicate to me that they are all wearing evenly & not one is failing over the other.
Still does not nessacarily mean that everthing thing 100% but shows even wear on the rings etc.
If your manual is correct & states 120psi is ideal & +-15% then 102psi then your under the factory specs, rings do wear & can cause engine failure.....fact!
Why would a manual not show the recommended PSI, stange comment imo??
Will it do it tomorrow, no, next week, maybe?
In the end you trust the mech's opinion or not!
If buying a second hand boat/motor I would think it normal & sensible if the owner cant produce servicing doc's to do these basic's minimum, water pump, fuel filter, gear oil, spark plugs, grease all moving parts & any other items that can visibly be noticed which could include carb service, thermostat.
My mate just had his similar era 60Hp Merc serviced & just doing the basics was $800.
Wasnt cheap but & dont think its unreasoanble either & at least gives you the confidence when you head out.