Lara,
One thing to remember before you mortgage your house,car and any kids of working ages, your camera doesn't have a full frame sensor, so what you saw through the lens, times the 1.5 sensor factor, was really what a 900 mm lens would produce!
So a much cheaper lens,with a 2x converter would get you into the same "ballpark".
As I mentioned above, I had access to the Canon version of this lens,for years and only probably used it 2-3 times and that was mostly just to play with it,
Rich