I have a fs250R trimmer just like the one you have and have cut 4ft. high grass and several 2" sapplings and even a little 3" maple with it using the three tiped brush blade sharpened well. 4ft. grass will work the machine hard and the operator as well. I've used the string trimmer for grass as well and feel the line trimmer works best on grass but for thicker stuff, the brush blade is the way to go. With the "R" version your not supposed to put a saw blade on it according to Stihl, only on the Bike handle version. Even with the brush blade you need a bigger deflector and a barrier bar attached to the handle. I'd run it with 105 "X" line and see how you like it before buying anything else for it.
For regular 8-12 inch grass the fs250 is way over kill and will do best just off idle. For tall stuff it will do it, but I wouldn't want to do over 2ft grass for more than a few hours. The machine will vibrate you so wear AV gloves for several hours of brushcutting. I've found the 250 is like a tweener saw, too big for small jobs and not enough for big ones. Still, it puts a smile on the face in shorter grass. For all day cutting acre after acre in 4ft grass, I went to a fs550. While it broke me in fuel, I wasn't tired after cutting several acres with it. And it was much faster as it didn't bog down.
I bought mine for the same reason a lot of folks are thinking about. I wanted it before they stop making them, and I'm not sorry I did.
I found one on craigs list last week for $200 and it was a year old.called about it the other day and it had done been sold.same for a rebuildable scag 52" advantage deck walk behind for $500 today. 
