i've built extensions with less work that that shed base
as sean says would have just shuttered a slab and plonked the shed on,job done in a couple of days
problem i can see happening is that the shed is gonna be so high,there will be a lot of wind shear
make sure its anchored down well
and before you do the path ask the experts,forget about *friends,neighbours and the guy with the dog that walks past every day*
a tip is only a tip if it works,if it doesnt its bad advice
IYKWIM
LLL
I put one of those sheds up earlier in the year and just cast a slab about 4 inches thick with an inch or two extra all round.
Make sure you have a very calm couple of days before you attempt to build it, as the frames aren't that strong until everything is in place and the side panels act as giant kites/sails. Make sure you get the base dead square, and be prepared to drill holes as a lot of the roof support stuff won't be properly punched out. A cordless drill and screwdriver bits is a good idea too - there are a feck of a lot of screws in one of those kits lol
