Tohatsu made all the 2 stroke boards for mercury up to 2005 from 90hp and below, I believe. The core motor design has been unchanged for years, with upgrades added along the years. They are the Japanese work horses, use primarily in saltwater conditions around the world.
I have owned a 25hp branded by Mercury since 2001 and I run the heck out of it, I have had to replace the fuel pump diaphragm about every 4-5 years. Maintenence is a joke. Very good power to weight ratio. I own a 2017 50hp tldi now, very similar like a big brother to the 25. It has injectors, sensors, on board computer, more tech. I have only put 12 hrs on it so far. The place I bought it from hails it's reliability. They told me that of the decades they have been saleing them they could count on their hands the number returned for warranty work.
With that being said, I am a maintenence freak! Bad thing about a used motor is you don't know how the owner treated the engine. Unless they keep a detail maintenence log with receipts like I do. Or they have dealer records of maintence. If you trust the dealer you are buying it from and they perform diagnostics on the outboard. You should be confident.