If you have rough ground are you prepared to buy in hay and feed for them over a bad Winter? Do you have some housing for when the going gets really tough or if one of the sheep has a problem? We normally only house for lambing but last Winter the ground was so wet (though not flooded) we got the pregnant ewes in in January for 6 weeks and only turned them out briefly so we could clean out the shed ready for them to come back in for lambing. It was hard work, and they romped through the hay stock for the first fortnight, but we didn't lose a single ewe, which is more than could be said for our neigbours who didn't house theirs.
Any breed of sheep will eat trees if they're hungry enough!