About the VGA/RGB bit: You won't need a ScreenBlaster. All you need is the correct adapter (Atari sold two of them, RGB and VGA) and then your system will run quite perfectly. ScreenBlaster (and the likes) are only needed if you require more colors/higher resolutions than the standard Falcon can offer.
Usually, an adapter comes with the used system (how else could the previous user use his/her system? ;)), but it's also possible to make one yourself.
I guess 400C$ should be enough to get you a basic setup (excluding monitor).
