Hi Ruth - I'm new this year too!
Your plot sounds huuuuuge! On sheds and greenhouses - rules about this should be in your allotment agreement. On my site, we're allowed sheds and greenhouses but they are very strict about the size.
I've only had my plot for under a month, but I've started lots of stuff in pots at home (beetroot, beans, peas, squash, cucumbers, tomatoes etc). Last week I found seed potatoes in a local garden centre, and they had a lot of winter veg plants too, so you should be able to plant quite a lot.