I have a Lienbach Line post hole digger with a 12 inch auger. I'm not sure what the model number is, but I bought it brand new with my Century tractor for $295. My dealer told me that was his cost and he wasn't making any money on it.
I would never own a smaller auger than 12 inches and have no idea what you would use 6 and 9 inch ones for. If you think you'd use the smaller one for 4x4 posts, you'd be mistaken. You could put one post in a small hole, maybe get two in line, but that's it. You need every bit of a 12 inch hole to get three posts in a straight line. Even then, you will need a post hole digger to straighten out and adjust your holes.
Thre are all sorts of things under the surface that you can't control, but will affect the direction of your auger. Roots and rocks are the most common.
For $700, I'd go to some dealers and look for a Lowe or Pengo post hole digger. They have special bits that you can replace or upgrade. The Leinbach has a lousy, cheap head on it's auger that will wear out in a few dozen holes. Then it's a nightmare to dig with it.
As for gearboxes, the Leinbach seems like a solid unit. Mine has busted dozens of sheer pins. I put a new one in and it works great until I break another. No complaint on that part of it.
For what your paying and what you'd be getting, I'd pass on that deal.
Eddie