We have an FS85 Stihl Pro and that thing is bullet proof. They do not make that model any more. Been though whack line and blades and 5 years of blackberry, scotch broom and hawthorn removal. Oh, and grass... miles of fenceline trimming and weed-whacking. It runs and runs and runs. I just tuned it up and it is running as well as it ever has (latest issue was a caked up muffler screen).
Scotch brooms are variable in size though. It will level the smaller and medium size brooms with one of several of the blade attachments that we have (we have and use 4 different kinds, from 4 edge, 8 edge, circular saw blades, and weird looking gnasher blades). Larger brooms here (2+ inch base) need chainsawing, or ripping out by the roots with the tractor bucket with a tooth bar mounted on it. We also use a lot of Garlon here. Only way to really kill the brooms and hawthorne and poison oak and blackberries. Spray and kill, then mow them down with the brushcutter or toothbar a season later (except the poison oak which I leave for a year before pulling and putting on a non-burn slashpile that is left to rot).
Note also that you can also get a chainsaw on a stick attachment for the Stihl straight pole brushcutters for about $100. I would recommend only a straight pole brushcutter, and preferably one with the bike handles so you can use it for a longer time. Note also that they are hard on your body; we have a 2 tanks of gas limit per day brushcutting rule using it here. The vibration is pretty strong, and you have to lever the things around with your body and here we have steep hills that we have to use it on.