Heater update - the company got back to me, and I can only hook up one of the heating elements if I want. Woohoo! I've got two 240V, 50A lines available already to check it out...unfortunately, one of those is my AC unit...and the other is in my kitchen. I don't really want to run hoses through my kitchen.
I don't have any 240V lines in my garage at all, which kind of surprised me. The third 240V line that I have goes to my dryer, which might work well - it's only 30A, but if I keep the load down, it should be fine.
I'm feeling better about this, that's for sure. I also realized that a number of houses, including both of my neighbors, have their electrical boxes on the side of the garage pretty near the water heater. Mine just happens to be on the other side of my house from the garage, which is a decently more involved install for the electrical lines.