I am building a garage .
Here is what I can offer .Codes will be in place for the foundation, ie-4 inch thick with a 8 inch thickened edge. I wouldlook at a garage package as it will give you the room you will want later. Costco does sell a plastic garage you may want to look at on thier site. If you do build a large shed, you can build a base out of pretreated lumber and use deck cement blocks to house the base frame (have an x in them). Electrical is another thing, you have to trench it in. Do it right, I have been to too many fires where it wan't built to code and no insurance and you don't need a city inspector nosing around Look at menards.com for a reference to shed sizes and prices-3k for a 12 by 18 with floor-good luck