I also run 87 with 10% (I think that's what it says on the pumps around here) ethanol and have no problems. I don't worry about fuel stabilizer either. I just shut the tank off after use and for storage I run the carb dry.
I wouldn't worry about rebuilding the motor until a need arises. You'd be shocked how many hours the iron Kohlers are good for. Just keep up on oil changes, use quality oil, and check it before each use. Trust me, the tractor will DEMAND other repairs soon enough. They can smell money.
Congrats on the new toy!