Bad luck on your problems, but rather than just looking at second had boxes, have you spoken to any transmission specialists to get a guide price for repair, at least then you know what’s below you rather than hoping the second hand box will last. I put a secondhand engine in my transporter T5 but could never get the comfort in my mind to trust it to go long distance camping, not that it ever broke down!
Your going to take a hit on it if you sell it as it is, yes it’s gonna cost to fix it, but what guarantee will you have on it’s replacement, and that’s more money again and maybe more jobs too
Cheers