[Sperkowsky]

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.