Used fatory LSD units can be "rebuilt" to new spec in about an hour, so thats not a deal killer in my book. You do want to make sure you replace the clutches before throwing it in, of course, as there is nothing quite like buying and installing an LSD unit and then finding out that it doesn't limit slip...
I looked into the Audi units before, they are not quite identical and I was led to believe that it is not suitable for our cars. Do you have more info you can shed on the subject? It would be great to hear from someone who has done that successfully.
Regards,