For a robotic "carry-all" that can carry 500km at walking speed, you'll need some serious power, especialy if the terrain ain't a factoriesmooth floor.
Use a roboteq for the added control and sensor inputs these allow.

You'll need serious grunt, and a lot of heatsink capacity, so we won't muck around with small high power but easely overheating motors.
http://www.robotmarketplace.com/prod...A40-300 motors.
Or else the more easely available ISKRA AME series permanent magnet motors (Hydraulic shops will have acces to these)
Batteries can vary between large SLA's and normal grade LiPo.