A quick update, for anyone who comes across this thread in the future. I did manage to get a Sigma 45 plus from CRES and have been using it for about 6 months in a commercial environment.
It's not perfect, the software and configuration options leave a lot to be desired, but is capable of exceptional results under ideal conditions. Limited parts and service remain available from Durst USA, and the service manuals I have are quite comprehensive. It's not too hard to take apart and put back together. The rollers on my unit are OK but if (and when) they eventually go I don't think it will be an impossible task to locate or manufacture a suitable replacement.
I would love to hear from any other Sigma users out there in the wild, it's a glorious machine and there aren't many others around.