The only thing you will want to watch for in a use pre-pro is if you need DVI, HDMI, and/or Component with video switching or not. If none of those are that big of a deal and you have 5 channels of amplification... you can probably find a pretty good used pre-pro in that price range. Otherwise I'd jump on that Denon 3805. I sold my 3805 just 5-6 months ago for $725 or $750. I used it just like you are suggesting and I use my Denon 3806 that same way now.
Here a few to check out at Audiogon:
Parasound HALO P3 B&K AVP-2030 (lol... ironically being sold by Hawthorne Stereo) ... a little over your price range but they might negotiate.
Acurus ACT-3 (another from Hawthorne)