Wasn't audi TT's reknown for having sump plug issues and most people would:
1) Helicoil a new one (popular and cheapest)
2) Weld and tap a new one
3) Replace Sump
Also check its not the bolts thread that are damaged, rather than just assume its the tapped thread on the sump to make sure. All jobs are sump off jobs due to swarf going around in engine, some people have not found it an issue, but its better to be safe than sorry as tech mentioned.