Does anyone know if and where the MMI informatic system can be repaired.

Something like that is impossible to diagnose over the internet. If the Audi dealer is telling you that replacement of the MMI system is your only option and you don't want to spend the money, then try and find an independent german car mechanic in your area so they can look the car over to verify if that is true or it's a simple wiring issue saving your thousands. So pretty much a second opinion in this case.