Most metal doors are gonna be pretty damn tough. I've installed a few that we're a tad flimsy but if memory serves this was because of a window in it, which yours doesn't have.
Another option would be your local commercial building contractor. They may have a good used one sittin around that you might snag up for next to nothing.
About all the commercial doors are steel with a styro core.

Any of these doors are only gonna be as flat as the stand ya build for them though.
Any surface wood or steel wil eventually warp to the shape of it's supports so keep this flat and you'll be fine.

If you wanna drive to Ohio I have 2 out back that I'll give ya for free.