The post about hours is important. I have maintained, tuned and rebuilt a bunch of Kohlers. While they are a good quality engine, if the motor has a bunch of hours, usually over 1400, in my experience, you get nickled and dimed to death. If this is the case, hours wise, get a short block.
If the mechanic is really a friend of yours, perhaps this could be an excellent learning opportunity for you. Have your mechanic friend watch you as YOU disassemble the motor, prep it for what repair is necessary, then reassemble.
I have done this several times with friends in the area. They always tell me the same thing, "I didn't know working on engines was so interesting" or similar words to that effect. It also helps to have over 10 grand in tools.
However, if time is the main criteria, short block it.
Good luck, Steve