Perry
I take it for that kind of figure it is a planetary reductuction drive? ( Not the old square box drive with a chain and sprockets inside) The planetary ones will follow a 12" bit down the hole for extra depth.
I've never had to rebuild my model 15c . However, we broke the output shaft off and that was a 650.00 repair. It has a small amount of greasy dust collecting around the output shaft now that is 8 years old.
Take the bigest bit that you will use, the larger the more they cost. Look for one that has replaceable teeth and has not been run with the teeth missing, wearing out the cutting edge.
I'd say if it will turn a large bit ok and low engine speed then the motor is not worn and bypassing. Of course warm the loader up for a few minutes too to thin the hyd oil down.
Ken