The 604ev (genuine Apple 604e processor daughtercard at 250MHz or faster) requires a Mach V motherboard from a late model 8600 or 9600. If you are running a 300MHz 604(ev) in your 8500, you must have already done that motherboard swap with an 8600/250 or 8600/300. So it will also run a 350MHz 604ev which can be easily overclocked to 400MHz. That would be a nice setup. I have a 400MHz 604ev in my 9600/350.
Dual processors are definitely possible, but I think you will need the original motherboard. I don't know if the Mach V motherboards will work with older daughtercards than the 604ev (3rd party upgrades are a different story). Any dual processor daughtercard should work fine from the dual 180MHz 604e in the 9500/180 MP to the 200MHz 604e in the 9600/200 MP. Daystar processor cards (nPOWER upgrades as well as processor daughtercards from the Genesis and Millenium series of clones) will work fine. I've got an nPOWER (dual 200MHz 604e) in a PowerTower Pro. If you are looking for a speed increase, don't bother with the dual processor cards. The classic Mac OS spends more time managing the two processors meaning that there's more overhead with a general slowdown as a result. OS 9 is better than System 7 in this regard, but it's still nowhere near as good as a truly MP-aware OS like OS X. You will only get a speed increase by running certain MP aware apps (like Photoshop). For general use, the MP setup on PCI-class Macs isn't very practical.
I think I would expect to pay about $50 for a 604ev/350 and probably somewhere near the same for a dual 604e daughtercard. But you'll only be bidding against other collectors like yourself.
Peace,
Drew