Jeff, that's a very good price for a lightly used D810. I stretched a few years ago to buy one (refurbished, which brought the price down, though not to that level). I'd love to upgrade to a D850 for the focusing system, which sounds as though it's a significant step up from the D810. However, used prices are still way up around $3000 and I'll be surprised if they come down anywhere close to your $1300 D810 opportunity for many, many years. Anyway, I can affirm that the D810 is a really capable camera. There's nothing it does badly.
Cheers, and good luck whichever way you go - John