... the number of people that have had issues with getting the 680's S/PDIF input to work correctly.
The issue that affects both 680 and D50 is that legacy gear like the UA-5 doesn't output a compatible signal. This means you need to do your homework first, to see whether your A/D is compatible or not. Let us know what gear you intend to feed the recorder with, and we can tell you what's a good match.
The 702 sells for $1,875 new, I don't think I've ever seen if for $1k used.
If you want to save some cash, you can get one of the older models used cheaper (671, HD-P2, FR-2), but if you have $1k to spend, the 661 is smaller and lighter while still being solid and reliable. I'd say get a 661 and be done with it.