Converting a Pole Barn
I myself do not like concrete coming into contact with anything other than pressure treated wood. Concrete is a conduit for moisture. With that said:
I would wrap by 6x6 at the level of the concrete slab (if the columns are not pressure treated) with some type of product like grace vicor or even ice and water shield. This will need the concrete from contacting the column.
I do not like concrete touching metal if also exposed to air (rebar and welded wire mesh is fine inside). I would fine a material to place between the siding and the concrete, maybe something like a bituminous fiberboard. As far as using the metal siding as a form, that would depend on how well the siding is attached. I understand you are not placing probably no more than 4 to 6 inches of concrete, but you do not want to have a "blow out" while placing your slab. Maybe some bracing along the outside bottom of the siding would aid.
make sure to install some control joints in your concrete, probably best at each column (don't know if you have interior columns or not), basically run a line from column to column (not needed along outside perimeter of slab). Typically 1/4 the depth of your slab is sufficient. All concrete cracks as your know, so this will allow it to crack where you want it. Just a friendly reminder.
my humble opinion, I'm sure you'll get many many more, and some will probably disagree with mine.
"You get what you pay for, and sometimes free costs more!"