The Bigma (50-500) is a very nice lens, one of Sigma's EX series. The newer 150-500 isn't part of the EX series and isn't as well-reviewed. I have and use the Bigma often. Even at the "wider" end (hard to call 75mm FOV "wide") the shots are nice.
To really save some money, the mirror lenses are fun and some of them give decent images. You'll have to decide if you like the ring bokeh/highlights, though.
Search KEH or the other used sights for the 500mm f/4.5. That will give you a lot of reach and a fast aperture.