.. spades tend to be a bit of a waste ... {and} ... shed invaluble especially for those quick showers
He uses a spade a lot - its good for digging in muck or making holes and trenches, more efficient (make sure your spade is long enough for your height, digging can be hard on the back! - he has a longer that average spade which he would never be without (he is 6'3").
A shed
is a good idea - get the best you can afford and line it inside with scrap wood as you can lay your hands on it. Cover the window with chicken wire on the inside. This will make it slower for any miscreants to interfere with - it will not make it completely safe - if someone wants to get in they will. Don't store strimmers and such like in the shed if you know your plot is vulnerable.
Poly - a fantastic pleasure, but wait a while till you are growing happily on your plot as you will have enough to worry about getting the other things sorted. Polytunnels cost (hers cost £400 and will need a new 'skin' probably year after next - depends how the current one manages over winter - some kids slashed it in May, but she repaired it with poly repair tape - not very good, Scotch outdoor tape is better, even though it has a greenish tinge). Also they get VERY HOT, even when it's overcast and they may need to be opened and closed. If you can't get there to do that, they should be left open all the time. This has its own problems.
Similar sentiments to a greenhouse on the plot, although they are good for starting things off (think about regular watering). Glass could be a problem is some miscreant decided to kick it in (like on our plot last May). Some Councils won't allow glass anyway.
Follow this link on this forum (or use the search facility subject: poly author: oliver) for a bit more information.
POLY LINK You could also try freecycle - they often have things 'free to a good home' usually you
just have fetch / to take the thing down - shed or poly - but this could be a lot more work than saving up and paying for a new one!
Good luck!