"Really big", "decent RPM", "enough load", "too much battery", and "spend too much" are about as vague as you can get. If you want some help you need to provide much more specific details. If you don't know them, then you have to plan it out more before you start asking questions.
In case you haven't though about it enough to realize it yet, RPM, torque, and wheel size are all related. You must know two of them to decide the third. Torque and RPM are almost meaningless without wheel size What matters is how fast your robot moves and how much force the wheel exerts (this is NOT torque, if you don't know the difference you must do a little more reading about moments/torque vs. force).
In order to figure out the torque and RPM required to move a certain weight robot at a certain speed, you must know wheel-size, and then figure out the speed you want and the force you want the wheel to apply. Then you muse use these 3 numbers to figure out torque and RPM. Either torque and RPM alone can't be reduced to utter uselessness by using giant or tiny wheels, respectively.
Larger wheels -> needs less RPM to move at a given speed, sacrifices force
Smaller wheels -> needs less torque to apply a given force, sacrifices speed
And how much can you spend? Just getting 10 sensors and webcam as well as a control board to handle them is not cheap- and the motors, mechanics and battery can EASILY outcost that by several times if you aren't careful or want super efficiency.
Are you looking for something like these an indoor flat-floored mechanically simple two-wheeler that are loaded with vision sensors and stuff ($$$) like this:
http://www.drrobot.com/products_wirobot_list.asp?categoryID=2
Or an indoor omni-directional flat-floored one like this:
**broken link removed**
Or 4-wheeled outdoor-capable ones like this (I have one of these):
**broken link removed**
What I'm getting at is it may be cheaper for you just to buy a platform if you don't know this stuff. UNless you go with the ones loaded with electronics eyes and stuff or larger 4-wheeled ones shown, the platform can cost you about $200- nice choices are the omni-directional one or the smaller of the 4-wheeled ones. One square foot or so is about as big as you need it to be. If you want something the size of a desk...well...be prepared to spend a lot of money and have it all go to waste , since it's pretty obvious something like this is out of your league right now.