Home depot has steel doors for <$150 (prehung).
They also have security metal braces for around the deadbolt... make sure you use 3" screws, and they go into something not air (space door good).

Instead of making this I suggest checking on craigslist and putting a nice shed on a concrete pad.

My folks scored a nice shed that was delivered on flat bed (used) 100% finished inside, with a loft, for <$3k.. T111 siding, comp-roof, extremely heavy duty. I think it's 12x16.