People put sheds in corners, not in the middle of their garden, so most are realistically going to be within 2m of a boundary so not usually allowed.
I would not rely on the whole "temporary" argument. That's often touted as the miracle solution by the people who are flogging the things, as it's not their problem if you run into problems after you've installed and paid for it. Best to avoid lengthy legal disputes, even if you think you know all the answers.
Besides, a shipping container will always look terrible however many planks of wood you nail on, or hanging baskets you screw to the thing.
Second-hand usually means there's a reason why someone's dumping it - it's probably beyond it's useful life, will have spent many years sprayed with saltwater at sea and is probably as rotten as an Austin Allegro underneath.