What is the most cost efficient way to build a mobile sound system for portable parties?

I am trying to start a club at the University of Illinois called Random Dance Party. What we are going to do is create a kickin’ boombox and travel around campus jammin’ out, gathering others as the night goes on.

My dilemma is this: (see question above)

I could either buy a home sound system or two, different car stereo parts, or just get some large professional speakers.

What should I do?

My concerns are: cost, loudness, quality, bass, and weight