My biggest issue with 4 strokes, used that is, is that you hardly ever know if it was serviced correctly or not.
Most owners buy a package deal (boat, motor, trailer) on easy payment terms - use the boat quite a bit the first season, don't have it serviced at all, then it gets out a couple long weekends until the significant other starts complaining about the rig hogging the drive/garage that's costing money (payments) and never gets used.
With a 2 stroke you clean the carb, service the water pump, throw in some new plugs and off you go. 4 strokes are far more temperamental and "needed" to be serviced in accordance with the schedule.
So to ensure you don't end up with 175 lbs of scrap metal, in short order, I would make a condition of sale a positive tech check. Paying the cost of having it checked by a certified tech is the only sure way of ensuring you are getting a functional engine that won't cost as much as a replacement in repairs to get it working properly.
Now, if the owner has receipts to show annual service or if it's a marine dealer and they are offering at least a minimal (6 month) warranty you can buy blind (but I would want to at least see it running - a test ride would be even better).
And as far as no hour meter, that is never a concern of mine. A 6 year old motor should have 400 to 600 hours on it - anything significantly less = usually lack of service since no one spends money on a motor they don't use or lot's of sitting around, which kills more motors than overuse.
