I'm not familiar with the C2, but an excellent used Thiel at that price point would be the CS1.5. This is a fine sounding floorstanding 2 way design. It is excellent with vocals and has wonderful timbre. I don't know what NAD model you have - if it's too bright it will not partner well with the Thiels. If you don't have enough current, you will lose bass extension. Thiels are all about detail. You haven't described your front end, but a pair of Thiels will ruthlessly expose every upstream shortcoming. They could cause an unwanted upgrade chain reaction in the wrong system. But, if you take the effort to properly configure a system for them, Thiels will thrill.
1K speaker advice?
I am looking to upgrade speakers from Dana Audio 1s to something full range with better resolution, with about $1K to spend. I have a NAD 100wpc power amp. I listen mostly to vocals, acoustic music, and blues - and to a lesser degree jazz, rock and classical. A local audio store has a used pair of Thiel C2 at $700. They sound great but I'm concerned about the age and how they will sound with my front end equipment. Am I better off buying something new - if so I welcome suggestions.