A reconditioned box could cost you £1000 if you can find one, replacing the bands is the way to go. Have you checked the condition of the bands and tried resetting them via the method in the manual? Can you see the threads on the adjustment screws of the brake bands?
If the gearbox has overheated you might have to change the fluid fly=wheel seal. I have a gearbox that has overheated in the back of my car its not a pretty site, the heat transfer can be seen on the drums (one doesn't want to come apart)!
Plenty of pictures here of the gearbox.