Toby,
Mark is an expert here. I would listen to everything he has to say, as he's living a very comfortable life, doing exactly what he wants to do, and not being exploited by language companies who set up a revolving door policy for both students and teachers. No envy here...
My only comment would be not to think of traditional brick-and-mortar institutions. A lot of language learning can be done online, and done successfully. I've started moving into this area, and it's not different than a small class of students. All the tools are the same, as well as the classroom opportunities. Plus online means you also have the chance to find students from all over the world.
Good luck.