Well first let me point out that you don't need FS2 in order to play Beyond the Red Line (the BSG mod) so if you're only getting it for that you're wasting your time as it will completely ignore your FS2 install and place itself in its own folder.
However FS2 is an excellent game in its own right so I'd suggest getting both. The link Mustang posted is a little old so I'd start with FS2
here. The installer gives you some of the best FS2 campaigns released so you've got plenty to play there before you need to go looking for anything else FS2 related.

You can find Beyond the Red Line
here. Have fun with both.