What is your budget?
Best raised beds, are pallet collars - 2-3 deep. Ebay usually £5 but ring round pallet suppliers and ask if they will donate any. Usually 1mx1.2m so a decent size to start off with.
Composting - make sure this is set up before you start
Soil - you need topsoil in the raised beds to start off with, otherwise it will take a fair while to build up enough compost in the beds to get started. Ask the local council if they will give you some of theirs that they make from the bins collections [eg Coventry will supply a load through bubbenhall at around £70 to pay for the delivery. and you get a huge truckload].
Have you got a mower for that lot?
Polytunnel, although growing in one is great, you need alot of water. I use them for potting on and gathering in the winter, and then around April/May start making the space into one for tomatoes, cucumbers and chills - as you move the seedlings out for the raised beds, the space becomes available for the tomatoes.
Have you got electricity? One of our best buys was a submersible pump, we collected the water from the roofs at one end of the site, and pumped it to water butts/wheelie bins to store it near where we would use it [at the back of the polytunnel].
As you have alot of space, leave a good space in between each raised bed for mowing purposes. All those mowings will make the base of some good compost.
It does sound exciting, I've set up loads of these sorts of projects, and the best thing to do is to not get too invested, but invested enough that it doesn't drive you mad. Prepare to do all the donkey work. If you get someone who is prepared to help then that is fantastic.
Whereabouts is it?