if you want to keep the car long term then I would rebuild the case instead of buying a used one (can be the same s..t).
2 months ago my transfer case was also off to change all seals, o-rings and during this job I have found not so difficult to even more open and rebuild the unit if needed. All the parts for rebuild cost about 100 GBP from Ashcroft and you can also find good instructions how to do it.
But if your intermediate shaft is leaking and moving a lot without O ring means the casing is worn and need to be sleeved or changed to new one.

