Unless you're willing to replace the hard drive yourself, the only Rockbox-capable device you'll be able to find with an 80 GB hard drive would be the 5.5G iPod Video. You will have to be very careful and make sure you get a used 5.5G and not a Classic, as the iPod Classic is not compatible with Rockbox.
If, on the other hand, you are willing to replace the hard drive yourself with one of a larger capacity, I can't personally make any recommendations but I've heard a lot of good things about the Toshiba Gigabeat players and the iriver H300 series as well.
As for file and folder management, I'm fairly sure all Rockboxable devices currently can be connected to the computer in a UMS or MSC mode, and most (if not all currently) can be connected and used in UMS mode while Rockbox is running.