Using a ladder as the frame would work. The problem with using aluminum is if breaks. Welding it take specialized equipment that not all welders have, and where as steel any ****er can weld it. You can always bolt it together, so if a bolt was to break its not too catastrophic. Go to
http://www.metalworldinc.com/
The have information on all the different types of metals, and the weights of them. That will give you more of a clue of what it will end up weighing.
On the plywood, if you are going to use tin or something to cover it, you can probably get away with a 3/8" plywood, or maybe thinner.