Steve, I guess times have changed, at least for washing machine motors. But i imagine it should be possible to find some older washers or dryers still? I imagine the giveway of what type you are looking at is the motor speed sensor you mention.
Fixed speed motors generally have a throwout set of contacts the changes the motor over from its starting windings to its run windings when it gets up to speed. They are also mostly skeletal as you say -- and look quite similar.
For the fan for your furnace, generally the two things you're looking for are capacity (you mentioned cfm) and blast pressure (in the same units, generally "inches of water column"). If you describe what type of furnace you plan to build I can probably figure what you need for those -- if you want. Helpful info would be the kind of metal you want to melt, furnace bore, and the fuel you want to use.
On the other hand, I'm sure your vacuum cleaner would work as a backup in case you can't find a motor to suit.