Funny I just found this thread... I'm about to do that very same thing... that is, build a reloading shed. I put up a little greenhouse at the side of the house for my wife, but she decided she didn't want it after all... so, it's gonna come down and a shed is going up.
I'm thinking 10X12, but we'll see how things progress...
Probably wood floor, 2X4 framing. If I bought a shed that size, it would be 1400-1800 dollars... uninsulated, etc...
I think I can do this one for under 750, but I haven't run all the numbers yet.
Should be WAY better, climate-wise, than the garage. Not too difficult to heat up the garage in the winter, but impossible to cool down in the summer.
Keep us updated... more pics!