Jack,

1) Never used the purple ones

2) I have used gold motors before and they are great little motors for a ramming machine. Only problem is that you require a gearbox, chain and sprocket arrangement etc which I was able to do in my pre-lathe days but it was a right pain.

3) I recommend buying some cheap cordless drills (around the 12V mark) and taking them apart. You can then screw a wheel on sold by robochallenge (either grant or james cooper) along with one of their drill mounts and you have a very nice reliable drive solution that will probably cost you around the 30 pound mark per side (depending on how much you spend on drills).

You can see these here fully assembled,

http://www.facebook.com/home.php#/photo.php?pid=140668&id=339100312http://www.facebook.com/home.php#/ph...8&id=339100312

And just the motors and gearboxes on their own,

http://www.facebook.com/album.php?page=2&aid=998&id=339100312#/photo.php?pid=36204&id=339100312http://www.facebook.com/album.php?pa...to.php?pid=362 04&id=339100312

or just add me on facebook and check out my albums.