I can get one of these in VG condition for a very low price. it's the Microdrive version.
My MTB is a mid-80's Bridgestone with a 6-speed freewheel, Sugino 46-36-26 crankset, and friction shifters. Will it work? I'm assuming of course that the bolt that attaches the thing is the same size as the original Shimano that's on my bike now. The original derailer works fine now, but you never know.
I never learned to ride a bike. It is my deepest shame.
Yes. It should work fine, unless...
(1) The Suntour rear derailleur has a short cage, in which case you may have some issues with chain length when cross chaining - possibly causing the chain to go slack in the small/small combo, or violently ripping the derailleur off the frame in the big/big combo - depending on how you select chain length.
(2) The largest cog on your freewheel is like 36 teeth (or something huge) that exceeds the max cog size of the derailleur.