$300 sounds reasonable for good condition.
You can run naks direct into the D6C or R1 on battery power no problem - you just need an adapter to go from dual XLR>stereo mini. Folks will probably tell you that you could do better with an outboard preamp as well. I used to run them both Nak300>D8 and Nak300>Zefiro InBox (an outboard A/D converter)>D8. I sounded maybe a little better with the Zefiro, but honestly, I'd give it a shot running in direct and then decide for yourself. Since neither of your machines has a digital input, an outboard ADC makes no sense for you, and preamp-wise, you could use something simple like a Church Audio CA9100. Supposedly the R-1 has kind of nasty/noisy internal pre, so maybe that would be worth it for you down the line. But like I said, just grab the mics first, then try it out. The Naks have a very pleasant sound IMO.