If its brand new (virtually impossible IMO), then I think its a great find. The DR10 was the last of the great cassette decks from the Nakamichi stable (mine is the DR3) and is definitely a single cassette, 3 head deck. It was more or less, an upgraded version of the DR1. If I recollect correctly, the DR10 was sold between 1998 and 2002. For it to be brand new, it would have had to be in its box for atleast 10-12 years. Given the quality of the rubber belts and rollers used by NAK, I am quite sure that everything should be in good condition (again depending on where the deck was stored). Give it a proper run before investing in it. 20k is a bit too steep by today's standards but this deck kind of costed nearly $1000 new, back in the day. Also, please remember that replacement heads for NAK 3-head decks are hard to come by. The 2-head decks from NAK are a better bet as their replacement sendust heads tend to be available.
On another note, the recordings made by the DR series are considered by many to be the most compatible recordings made on NAKs. Traditionally, audiophiles noted that recordings made on NAKs sounded the best, when played back only on NAKs. This myth was dispelled by Nakamichi with the DR series. However by then, CDs had taken over, signalling the end of supremacy for cassettes as the popular media of choice for music listening.