RE: Using MDF in construction...
I've only been building for a year or so. But I wouldn't do it. Plywood has long fibers that are layed up 90 degrees to each other to give it strength. MDF is just wood particles pressed together with some kind of binding agent. I'd be afraid the MDF would crumble under any kind of stress. And I know that MDF will turn to oatmeal if it gets wet.
I'd look a little harder at some more hobby shops for the model aircraft plywood. If you're desperate, as a last resort I'd use luan plywood from the local hardware store. They use it as an underlayment for flooring. It's less than 1/4" thick. Not as strong as aircraft plywood, but alot more resilient than MDF.
Hope this helps...