Daniel,
If the action is in good condition, it should feel nearly as good as the current offerings from Yamaha.
We've come a long way in sound development primarily due to the reduction in memory prices. While it doesnt sound bad, a P120 (or similar) will sound much better. Then you should consider other features like polyphony (PF85: 16 notes, P120: 64 notes), available effects, etc.
Regarding the value, the best thing to do is call a dealer and ask what he would give you in trade toward the purchase of new digital piano, taking into consideration a modest markup, when he sells it.
Hope that helps.
Regards,
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Steve Deming
Assistant Manager
Customer Support Dept.
Pro Audio & Combo Division
Yamaha Corporation of America