You're always better off rebuilding your own. You know that it will fit, your clutch will mate up correctly, and you'll know that it has been done properly.
The hard work is in removing and refitting the box, which you'l have to do anyway. Stripping and rebuilding is simple, so long as you have a manual (Haynes or better), and you're methodical. It'll take a day if you have the parts ready.
www.legs.co.uk sell overhaul kits mail order for less than £100, comprising all the main box bearings, bushes, shims, seals, gaskets and synchro cones. They stock all the major bits, so they can send them straight out if you find any damaged components like shafts, gears or synchro hubs.
The transfer box is likely to be in good condition as it deals with less torque and has heavier components, but it would be worth reassembling the intermediate cluster (removed to separate the main and transfer boxes) with new roller bearings and thrust bushes.
The likely total cost is under £200, unless you're unlucky, and is a far safer bet than a second hand box of unknown condition and history.
