I have owned a second hand 2001 Merc 25HP 2 stroke for 10 years. It is one of 3 other motors from 6 HP to 250HP. This motor has a cult following for a good reason. My 25 HP Merc has been bullet proof. All I have had to do is change out the water pump every 2 years and the lower unit oil once a year. This year I finally cleaned out the carb and picked up 3 MPH on the top end. This motor is easy to work on, so I have been able to do the work myself. I have the 25 HP Merc on a LT25 and I get about 10 MPG.
As far as the 2 vs. 4 stroke debate I plan to run the 2 stroke Merc for a long time. If I was going to get a new motor I would probably go the 4 stroke route to get better range. Four stroke technology has come a long way and is reliable. As far as hole shot goes 4 stroke performance is good enough for me and has not kept me from getter to where I need to go or out of a place when I need to go home. Bottom line between 2 or 4 stroke I don't think you can make a bad decision.
If you do buy a used 25 2 stroke Merc check engine compression, should be around 125 psi for each cylinder. Check the lower unit fluid, should be clean and not milky looking. Its best if you can take it for a run. Running the motor on the water at speed will tell you more about the motor than just running it on a hose in the driveway. For instance a stalling motor when giving it full throttle will tell you if you have a fuel issue like a clogged carb. Look at the prop and skeg for damage, that will give you an indication of how it was abused or used.