Craig,
DO NOT BUY NEW!
Look to the rental companies that have the tow-behind units and see it there are any that are coming out of service. I bought my first one from a company that did the whole fill it with cement, you drive it home, pour, return. It looked like hell but it did a great job in creating a portable ball mill.
I learned quickly to not use the paddles in the mixer, I cut them out with a torch and bought some huge steel balls and blocks to toss in. Next was mounting an additional pulley and electric motor.
The end result was a killer gas and electric powered portable ball mill that would turn the worst cemented material to powder in pretty short order.