There seem to be a fair number of used units out there for the original model at dirt cheap prices. How do they compare to the new one? Is it worth holding out?

How do the specs compare? Newer models often have better battery life, lower noise, etc., but that is not assured.

Used gear often carries some risk, especially if a private sale w/o warranty.

A post from a couple years ago.
https://www.gearslutz.com/board/so-much-gear-so-little-time/1000470-tascam-dr-680-vs-mk-ii-worth.html

I have not used either model.

As Don stated, better battery life, slightly better preamps and some other fixes and annoyances that were addressed with the original machine. Is it worth the added cost? That's up to you.
Note: even with MKII's better battery economy, an external battery system / power supply is recommended.
If you're running a bunch or wireless or other line level inputs, the preamps are of less importance, since the 680 is one of the few machines that completely by-passes the mic preamps (in line level mode).

Thanks Rick. Was going to feed it with a Mackie mixer. Not sure which has quieter preamps.