I'd really think about it before you commit to building the house (if you can call it that). You mentioned 14' x 20' which equals 280 sqft. if my math is right
. My bedroom is more than twice that with the master bath included. Do you plan on having a seperated bathroom and kitchen? What kind of floor plan do you want? Even my 1 bedroom apartment in college was about 750 sqft. Do you plan on having running water and electricity? Are there even connections for water, sewar, and electricity? It sure sounds like a hell of a lot more trouble than it is worth in my opinion.
If you are really on a budget, get a trailer. You could always reinforce the locks on the doors. You could probably even change out your glass windows for lexan. Overall security wouldn't be great since the metal walls are pretty thin, but how many break-ins actually involve the bad guys going through a wall?