I see you are going PRETTY BIG. 
Would it be easier to get a Garden shed, and do the conversion??? Sometimes these can be found on Craig"s that peeps just want little ca$h for, just take away.
Nor sure of your budget, but consider one of the Vinyl Sheds. Sams sell an 8 X 10 for about $1000. Will last for long long long.
You would just need to redo interior and add some ventilation.

Would it be easier to get a Garden shed, and do the conversion??? Sometimes these can be found on Craig"s that peeps just want little ca$h for, just take away.
Nor sure of your budget, but consider one of the Vinyl Sheds. Sams sell an 8 X 10 for about $1000. Will last for long long long.
You would just need to redo interior and add some ventilation.