Blackriver 
use a tread mill motor. i have seen many variable speed drill presses, milling machines, belt and disc sanders, lathes etc made from these. they got the muscle for what you want without burning them out and include a free mounting board, belt and hardware.
If you want the variable speed you will need a fully working tread mill with good board. the advantage of these is you can use them for about anything you would drive a belt for in the shop limited only by your imagination.
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