The pigs will avoid the nettles and brambles, but if you strim them back they might keep them down. I'd be a little concerned about the rooting around the trees, but if you have established old trees or can protect them they should be ok. The fruit bushes will need protecting or they will get trashed!
Pigs can be quite hard to contain, especially as weaners. They can quite easily lift stock fencing up with their noses and wriggle under, and they are impossible (almost) to catch when they do escape! I like to use electric fencing to protect the fencing you have.
They will make a heck of a mess over the winter and unless you drain exceptionally well, you will end up with pigs up to their bellies in mud. I would buy a trio of boys and aim to keep them till around 6-7 months and see how you get on. As they clear the vegetation they will become more reliant on pig nuts and when they get to around 6 months its very expensive.
I would buy yourself a good pig keeping book and read up on what you'd need to provide them with and the set ups and legal requirements.