Re: How do I hook up the motors to the wheels?
 

You may want to consider starting smaller in case something doesn't go the right way. Gearboxes especially can run hundreds of dollars even for just a drivetrain and arm. Smaller also lets you use hobby servos for rotation.

Re: How do I hook up the motors to the wheels?
 

We will need more information to help you. To start what is this for? Are you part of a first team? What kind of "arm" are you talking about? How many degrees of freedom is the arm going to have? What does this arm need to do? How are you manufacturing this robot you mentioned aluminum but are you going to be using sheet metal, tubing, extrusion ect? What exactly is your budget for the entire project? Do you have a control system in mind? What surface will this robot be driving on?

I think you should brush up on some basic robotics knowledge before undertaking this project. If you continue your chances of success are a shot in the dark which is not at all what you want especially if you are on a tight budget.

Re: How do I hook up the motors to the wheels?
 

This helps!

That's more or less the size class of a FIRST Tech Challenge robot (their size limit is 18" cubed). That says to me that a NeveRest motor is probably the scale to be using for this. Not that you can't use a CIM or other motors, but they're probably going to be overkill.

I can't say for certain without seeing the design, but I'd bet a few judiciously-used NeveRest motors and some 1/16" wall aluminum shapes (okay, maybe a little 1/8" in high-load areas) would get you pretty far. :)

Re: How do I hook up the motors to the wheels?
 

I think if you have an FRC team near you, you should go visit them and see if they'll let you just pick through their boxes and see what they're willing to give you.

My team still has boxes of mismatched transmissions, Power Wheels transmissions and old motors that I'd never put on a competition robot, etc. If I knew a kid was going to actually use them, I'd be pleased to give them away.

The mentors and seasoned members can give you advice too. Way better in person than on CD.

Good Luck!