1. Can a sump pump handle sitting in caustic?
2. How much liquid do you think I need to have in the system to run one keg at a time?
3. Will it actually drain effectively?
1. Kind of. We go through one every 3-4 months or so. Like Keith said, though, I don't think anything you can find at a hardware store will have the head you need for cleaning.
2. I'd say about 1/3 the volume of the keg, plus whatever you need to fill the lines. If you're gravity draining you'll need more because the kegs will get full.
3. Not really. You'll want to use compressed air unless you have a lot of time on your hands. At the brewery I used to work at I cobbled together a keg cleaner and it took FOREVER to drain using gravity.