I acquired a virtually brand new Wickes 8x6 shed when we cleared my sister's father-in-law's house.
The quality is pretty atrocious. The floor panels were sterling board and were badly sagging between the joists, the roof was warped. When I reassembled it it was like the proverbial old broom that had three new heads and four new handles. I replaced the floor completely with thicker sterling board and upgraded the joists with some old 4x2 I had. I framed the roof properly to square it up. It is now quite sturdy but I wouldn't want to lean anything too heavy against the walls as they are framed with 1"x 1" timber, the cladding is feather edge that is so thin it is warping. It is water tight but you can see large gaps if you look downwards from the inside. As it is a 'sitting shed' rather than a storage shed I will probably line it in the winter to stop any draughts.
When I erected it on concrete blocks I came back the next day to find the wind had blown it backwards three feet off the blocks it is so light. It is now firmly anchored with two 3"x3" fence posts in the ground on opposing corners. These were planned to be used anyway with pig-tail screw-eyes going through the wall into them from the inside as hooks for my hammock.
If you are handy with a hammer and saw I would build your own from scratch. Plans? What are they? Just have a look at the construction of a few sheds and copy/modify to your requirements.
I did so at home with my 16'x12' shed. Proper job with ship-lap cladding, the framing is 2"x1" for the walls at 2' centres and 2"x2" for the ceiling. Only problems I have had is the roof got damp when the felt roof deteriorated due to the acidic drip from the yew tree that overshadowed it which would have happened with any shed I put there. A few timbers 'sprung', I used a pneumatic nail gun and some of the nails didn't hold well, I would use proper nails next time although the nail gun made the cladding very quick and easy to do alone.
I started with a rough idea of the size I wanted and then based it on whole sheets of sterling board for the floor, minimising cutting or wastage. The cladding has little wastage as you use all the off-cuts around the windows. The roof is the only awkward bit as it is then slightly wider than the floor. Don't skimp on the floor, it's fairly easy to replace the roof if needed but the floor is a different matter. I built the walls as panels then bolted them all together, it would probably be easier to construct the frame completely then clad it once erected.
I planned to put perspex windows in but have never got round to it. I made frames then stapled UV stable polythene to the frames then slotted the completed frames into the openings, I have replaced them once in 10 years but they do the job nicely.