John, I just bought a Canon 70-210 FD for $40 that is very nice lens. I also just received a $60 Canon 100-300mm FD I have not had a chance to get out and test yet, since the rains have finally come to Los Angeles. I got both for my A7R. The 100-300 looks good indoors, for what that is worth, and works a treat.
I love the old push-pull zooms in locations without a bunch of dust. Dusty areas, probably want something else as the push-pull does suck a lot of air (and dust!) inside the lens every time you zoom. This 100-300 will get you out to an effective 450mm. You can also buy an "L" version of this lens for under $500, though how much better I am not sure. Both versions are f/5.6.
This lens will cost you under $100 with proper adapter and shipping! Going longer than this will start to cost you money, but it can be done just as easily. Remember too that since you are using an APS-C size sensor, you will only be using the sweet spot of the lens if it was originally built for full frame 35mm film. It is a lot easier to get acceptably good long glass than it is to get ultra wides.