You can always try the extension cable, and then build the shelf if that proves unsatisfactory. (Do not spend a lot of time and money trying different cables, however; build the shelf.)
The NAD does not have a ground wire, by the way. I bought one some years ago, and the lift mechanism was overlubricated, which meant the stylus never quite got all the way down. Don't know if that's still a problem, although it's quite fixable if it is.
For what it's worth, I tried a different Thorens before I bought the NAD. Your dealer might make more money on the NAD, but that doesn't mean he's not right.
The NAD does not have a ground wire, by the way. I bought one some years ago, and the lift mechanism was overlubricated, which meant the stylus never quite got all the way down. Don't know if that's still a problem, although it's quite fixable if it is.
For what it's worth, I tried a different Thorens before I bought the NAD. Your dealer might make more money on the NAD, but that doesn't mean he's not right.