Hi, this is just to bring the topic up to date.
I bought the s/h CVT box inc torque converter for £250, and had it fitted by an auto transmission specialist in Gloucestershire. All seemed OK, and £650 later we collected the car and it ran well, no noise, very smooth. Couple of weeks later we noticed a transmission fluid leak, not much, just a few drops on the drive overnight.
The car went back, and we were told that the leak was from the fluid cooler fitting (where cooling pipes connect), so for 2 hours work they swapped it over for the one off the original cvt box (which wasn't leaking!). A little later, still leaking, back to garage, this time the proprietor said it was just fluid that had collected in the under tray, which they cleaned off. I had my doubts (eg surely they would have noticed when work was done . . .), but I accepted the explanation - no charge this time.
You've guessed it, still leaking!! Took the car back again, garage not happy. They said they were not responsible for second hand parts, even though they had willingly agreed to do the gearbox swap in the first place. I pointed out that I didn't expect them to take responsibility for the s/h unit, but would have expected them to check for leaks after the work was done (3 times!). I just have to wait and see, hmmm. From what I've read a cvt fluid leak can be a number of possibilities, most likely the transmission pan gasket, or the cooler lines/connections to the cvt radiator, or the torque converter, or the internal seals. Not sure how an external leak would be apparent from the last two, though.