Stay away from a metal shed. The condensation build up it terrible inside. Mine rusted the hell out of my garden tools after only a few months. I'd hate to put my pride and joy in one. I also wouldn't like to have windows in a shed I was keeping a bike in. It only takes one set of prying eyes...........
Mines been fine up here in Mossley-Under-Cloud. It's ventilated. 4 years and no condensation, no rusted tools, no mildew on camping gear, even my 2011 Hollyoaks calendar (still reads April- Sabine ) hasn't wrinkled, curled or grown mould!
first kill the weeds, clear the shrubbery, dig out two tree roots ( if you are unlucky ) and then you can level the blocks,,,,