The mini GF is a great project. You will need 6mm (1/4) for the GF chute and a small bit of 12mm for the firebox. You only really need 1.5 mm to 2mm for the inside cook chamber and 1.2 to 1.5 for the outer skin. The thing is insulated very well. Which is another thing you'll have to look into... getting some form of insulation. In Brissie there is Bellis. That's where I get my insulation from. For insulating the inside of the tubes on the GF use bog standard fiberglass bats. One of the hardest things I found when building my GF is getting expanded metal.
So, if you are reasonably handy, can weld or know someone reliable who can do the welding for you... go for it. It's really addictive.
As for your missus not understanding you.... you are on your own, but welcome to the club.
![Rolling on the floor =))](./images/smilies/24.gif)