Goats are perfect. They will eat all the brush, poison ivy, etc. and trim all the branches as high as they can reach. They also will eat the bark of some trees, so protect anything you really don't want them to get. In a big pasture they won't do much to the trees, not for awhile anyhow. They will not however keep the grass down, and I don't think they care for thisle. I believe they will take out kudzu though.
They won't eat the thistle to the ground but often will eat the flowers off which keeps the thistles from spreading. The hard part is keeping them in and stopping them from eating things you'd like to keep.
Goats are also a huge p-i-t-a and you will need really good fences to keep them off your front porch and off your cars, really. I would hesitate to get them unless you plan to have them clear your land for massive planting, then putting the goats in a penned area. They do eat grasses when everything else they can get to is gone, but sheep make much better lawnmowers.
We kept ours in a double strand barbed wire electric fence, and very seldom did they get out. They stayed in the same fence with the ponies. The bigger ones seem to be less trouble than the pygmies though.
Thanks for your help, guys. I did a full walk through of the property in question and I think it would probably be a challenge for a whole phalanx of goats. A 1/3 of the property was marshy and I can see now why it was so cheap. I'm now setting my sights on an old horse farm with mostly pasture; I guess I need to get some horses now . I imagine those are more of a full time investment though. Maybe I can raise a herd of guinea pigs and hamsters.
They also don't like to get their feet wet, so they might not touch the marsh area. If you can mow that area, they could use the hay from it though.
They won't clear it overnite, but a dozen goats will clear quite an area over a season.
Do not get un-neutered males, unless you want one for breeding. They are filthy, stinky, and can get mean.
Yes, they will climb on anything. Dad left the manure spreader between the barn and the chicken coop one day, and one of the goat kids managed to go all the way to the top of the barn. Dragging it down on the ladder was quite the challenge.
They are herd animals, and will stay together, if we had one that was inclined to get out of the fence we got rid of it, and had few problems with them.