Yes, $700 is fair used. You would probably just have to update the firmware through a USB download. Make sure the seller has the original box, packing materials and manual so if you wanted to sell it at a later date.
With mine I don't intend to keep it as Wyred 4 Sound let's you trade in or should I say trade up when bought directly from them. I agree with you that the dac is the weakest link, but it's built like a tank with shielding and other positives and at the time I just needed a reliable head unit on the cheap and usually in those circumstances the dac is what it is.
Even though I have thousands of cd's and hundreds of 'albums', as I said in an earlier post I am a crate digging needle dropper and I mostly use my turntable and can always upgrade and invest in a separate dac at a later date since I already own a cd transport. And that's exactly why Wyred 4 Sound made the mPRE the way it is cause it is essentially a head unit/pre amp 'starter kit'.