We used an M10 rod and having lifted our sofa at least 3 times and been in 10 battles, the thread is good as new. Depends on your ratios I suppose. We could lift a featherweight comfortably.
As for drive, I'd agree that drills are a good starting point. We used (and will use) four 12v cheap drills with 100mm wheels, over-volted to 18v. The result is plenty powerful, and it can do 10mph.
If you go the drill route, I can't help but say, get all metal gears for the gearboxes. We didn't, and this happened:
http://s1088.photobucket.com/albums/i32 ... 012402.jpg
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