We had a commercially built waste oil heater in a shop that we sold a few years ago. It was great, BUT you had to run it more or less all the time for it to keep working right, which meant we were always scrounging for old waste oil, even though we did oil changes in our shop. If you let it sit without running for a while, it would clog up and you'd have to work on it before it would run right again, which meant that it was fine in the winter, but not so good during the summer months when it didn't need to run as much. Also, you really had to keep up on things like filter changes and cleaning out the exhaust.
Be careful in soliciting supplies of waste oil. Most people who just want to get rid of their used oil from their car are okay, but beware of people who run heavy equipment, etc, and want to get rid of a lot of it. There will probably be SOME waste oil in that 55 gallon drum they want to give you, along with antifreeze, diesel fuel, and whatever else they dumped in it. Now their waste disposal problem is yours. The guys who bring you waste oil in the jugs that their new motor oil came in are usually pretty reliable, as are guys bringing 5 gallon buckets. They've probably got a boat with twin engines, but again, beware of people operating commercially (fishermen, tour operators, etc) because some of them will fuck you.