Hi Cameron,
I'll do my best to answer your questions, but I'm sure you'll get lots of advice from everyone

For 2 weaners, raised for meat I'd think round about half an acre would be plenty space for them.
I see no problem with using a shed for a house - BUT it would have to be very robust - they do love to rub against things ! Does it have a solid floor ? You'd need to provide plenty of straw for them to lie on and I'd maybe consider halfing the door, so that only the bottom part was open (to cut down on draughts and heat loss)
Fencing - doesn't need to be high (unless you're getting flying tamworths lol!) about 2 feet high should do it. You can use electric fence - double line of the tape style stuff, one line about 6-8 inches high and the other at the top. Or standard fencing, but it might be worth running a length of barbed wire along the bottom to discourage them from trying to get under it. Whichever you choose - make sure it's sturdy and check it regularly to avoid escape attempts.
If you wanted to plant things for them, you'd need to wait til the plants were grown before putting the pigs in or the plants wouldn't get a chance to grow. But stuff like carrots, turnip, cabbage all go down well. But they go down equally as well from a bucket or trough, the pigs will root up all the ground so I'd think maybe your crops wouldn't get a chance to grow. I've not tried it - so can't be sure.
Make sure that you give them a wallow too - either a mucky bit of ground that you wet regularly for them, or a custom made one - they just love their muck

You sound like you're doing your homework

I think your pigs and chickens could live together more than happily in your orchard, just make sure your trees that are less than 10 years old have some form of protection from the rooting pigs. If they ate all your windfall apples you'd get pork with inbuilt apple sauce

Just got to decide which breed you want now - that's the REALLY hard part

Good luck and keep us up to date with how you're getting on.
Karen