A lawn mower would be the perfect application for a brushless motor. They are more efficient that regular brushed DC motors, but they need special speed controllers. Here are two brushless motors which would probably work just fine, most likely with just a single reduction off the motor shaft via timing or V-belts.
http://www.hobbycity.com/hobbycity/s..._270Kv_/_2100W
http://www.hobbycity.com/hobbycity/s..._170Kv_/_3250W