I am in a similar situation but I own a boat now and often consider upgrade to a larger boat. My 02 Ranger 518vs with 200 Yamaha HPDI has about 700 hours on it now, I bet 90% are at about 3000 RPM or less. Likely 75% are at idle due to scanning and the like. Those 700 hours have been put on it May - September (occasionally October) since 2002 when I bought it new. Always stored in a garage. I debate the same thing you are when I consider a newer bigger rig. Don't have much to add but to say, my 200 Hpdi, comes out of storage every year, gets batteries put back in typical measures to ready the boat, I back it into the water and she fires and runs flawless. Runs so good I cant get myself to sell it. Thinking repower will be the direction I go someday but not sure when that will be.

I think you can tell a lot by the shape of the rig. IF the boat is super clean and meticulous, up to date from a technology standpoint the motor has likely been well cared for. when you are shopping, check out the nooks and crannies. A boat can get clean for selling but if the bilge and other deeper areas of the boat aren't kept up you probably have a sing the motor isn't either.