Hi Samuel,
1) Building one usually works out mroe expensive, but you can end up with a much much higher quality building. Most of the cheap pre-made sheds are made with really thin
wood, and building one (there's various plans on the net for sheds made from old
pallets and things) could result in a much mroe robust and long-lastign structure (this may also depend on your building skills though...)
2) Put the shed on blocks? Either raise it up a bit so the wood isn't in contact with the damp earth, or put a conrete floor down (which probably won't be allowed on an allotment)
3) Polycarbonate windows are much harder to break- they don't shatter like glass. You could also have a ridged polycarbonate roof instead of windows? Our
chicken coop has this. It means you still get light, and depending on how you build the shed you may not be able to see the roof much from the ground.
4 & 5) In our case a really hefty fence/spiky hedge works.. but our allotment space is only 100m from the house and nowhere near any footpaths.. so I'm not much help with this, sorry.
6)
Solar lights! Its not very often we're down there in the pitch black or in winter, but the
solar lights recharge quite well in the summer and give
enough light.
7) Fishing/camping chair, and a camping-stove with a kettle! To be fair our chair doesn't get much use, I mostly just sit outside on the ground (I am possibly the only person who goes down the allotment to read a book), but a kettle and a stash of tea bags goes down well. Milage may vary if you like your tea with
milk, or have no potable water (we have a 25litre jerry can that gets filled from the nearest tap periodically- its not killed me yet)
Enjoy your
gardening space!