Seeing as you are used to cycling, going from a bicycle to a BMW 1200GS would be like going from a Go-Kart to a Humvee...
You have travelled on bicycle so I guess you are used to going at slow paces, enjoying the scenery and having low weight and low maintenace machines..
Well, a 1200 is very heavy, very fast, very complicated and unless you are very skilled on 2 wheels, its a huge handful when the road runs out.
I think you should try something smaller, especially if you want to explore the trails and paths you were used to on a bicycle.
If you want to go to South America, I would avoid all super high tech BMW's. The reliability just isnt there and the support you need just isnt available once you get into central and south America.
Ask youself.. Do you need a bike that does 130mph + ??????
Sadly, in the US, you dont get the XT600E, Africa Twin, Yamaha Tenere but you can get these..
DR650 SE: Will cruise at 70-80mph and handle the trails.
KLR650: Cheap and cheerful and also good highway speeds.
DRZ400
S: More offroad orientated, will cruise at 65mph. Needs more maintenace.
If your heart is set on a BMW, by all means make sure you get a test ride on one and feel the weight and make sure you are comfortable with it.
P.S... Avoid the F650GS's.. They are unreliable and badly made junk.
Btw, there is loads of info about bike choices etc in the forum.. There is a section called "What bike" which you should look at..
Oh, and WELcOME TO THE HUBB