I don't know where you are in the country but def look at a proper shed supplier - DON"T even think about getting some cheapo thing from the local big chain DIY place. I have both. One beautiful shed, which is tough, watertight, and has withstood one move of house. It came all treated, and in large pannels ready to easily assemble. I bought a cheap 'summerhouse' from DIY place, and it was a pain to assemble, fell to pieces, had to be added to to stop it rocking from side to side in wind, needed lots of preservative and still lets in water. And it has warped. Lesson learnt, you get what you pay for. Check out Burtenshaw sheds, Kent.
Posting on behalf of friend - have namechanged to preserve anonymity (!)
She is looking for a new shed. Has the foundations already but with near zero DIY skills really looking for either a shed that pretty much builds itself or a recommendation of people who supply and build sheds.