The addition of the overdrive is a wonderful thing! You know that you'll have to change the length of the drive shaft. You know that you'll have to change the speedo as the turns per mile change between the regular box and the overdrive. There was a relay and vacuum switch associated with the original installation. Personally, I believe that the switch and relay are overkill -- you don't really need it. Make sure you fuse the overdrive right at the switch so there is NO change of a dead short.

As for the source of a good, rebuilt unit? I have no suggestions other than watching and waiting (which you've already done, I'm sure). That three synchro box is always faulty -- either the first gear teeth or the second gear synchronizer -- so I would plan to rebuild the entire gearbox when you get one.