First, if you disassemble a used light fixture, the removed parts won't be purchased by any LFS at any price. You can't trade in a used ballast for new parts.
Any retrofit kit that you can fit into the old fixture housing will work fine. You might have to attach the new ballast to the outside, on the back of the old housing, but the result will be a good, workable light fixture. I would buy a ballast designed for T5HO bulbs long before I would consider using a used PC ballast for T5HO bulbs, even though it might work fine.