I've thought of a steel frame with compsite deck boards too.

Kiln dried should not up the price. Most lumber has been kilned dried by the time it leaves the mill.  I worked at a stud plant for a few years and it was kilned when it was rough cut before it even got to the plainer that cuts the final dimension.  That's a good point though choose nice wood to begin with.  No warpy wete stuff. The angle iron and carraige bolts will keep everything in line over the years as far as moisture over time goes though.