Okay--I bought a sink.
I debated whether to get a giant one or a small one. I frequently like to just go all out and get what I think I will want in the end, so I was tempted to get the one I found that would hold about 35 gallons, but ended up going with the one that will hold 15 gallons. What really convinced me was that I am not ready to do such big makes yet, and it would be really difficult to do a 5 gallon batch in the big one (only about 1.6 inches of milk with 4 gallons).
I had a great idea of how to heat 35 gallons of milk, though--I was going to get a plumber to put T's in the piping to and from our water heater and valves that would let me isolate the water heater from the rest of the house. With the T's, I would open a loop that would take water from the water heater, put it in a water bath around the vat to transfer heat to the milk. A pump would remove it from the bath and push it back through the water heater to bring it back up to temperature, and the cycle would repeat. I still would have had to heat up all the extra water, but my hot water heater puts out a lot more power than I can get electrically, and it costs a lot less to heat with gas than with electricity.
For now, though, since I got the small one, I think I'll stick with the thin heating elements. I don't need as much power and I think it would cost more to get a plumber in here than to just buy the elements. If I like this one and decide I need to make even bigger cheeses, I'll get the bigger sink someday. When I get this all set up, I'll start a new post and show you all what I've done.