I just bought one of these and it looks to be a very well built motorcycle. NSU was the predominate motorcycle builder in the world in the 1950's until Honda came in and literally stole their design and built the early Honda Dreams. The big 500cc single was built as a sidecar hauler and had a good reputation for strength and reliability.
The bike I bought does not look this good but is a very complete bike. I will post pictures later this weekend if I get time. The NSU is for sale as I am still trying to get the resources together to finish up the BMW R69S deal. I will be asking around $5000.00 for it, restored bikes are selling in the $12-15K range.