Hey man, if you can swing it financially, use this motor:
http://www.robotmarketplace.com/prod...G-S28-150.html
It puts out 3HP at 24V but weighs
less than 4 lbs. The price reflects the CNC aluminum can and the use of Neodymium magnets. My BattleBots team used them for a spinner-type robot back in high school. These motors rock! The downside- your batteries will need to supply 100A to realise the 3HP rating. I suppose you could mount a 3F car audio capacitor in there to help with acceleration....