hi Barney,
I would first recommend that you learn as much Spanish as possible, before arriving....if you have to, postpone your trip until you have at least an intermediate level of Spanish (i.e. being able to speak comfortably about simple topics with other Spaniards).
Though you don't have to speak Spanish in order to get an English teaching job, it will really help you whilst in Spain and your life will be much easier.
Regarding teaching English, I don't know how much exactly you could earn in Cadiz. I dont think it is one of the best markets for teaching English in Spain.
In Madrid, you can expect to earn 12-14 Euro teaching at an academy, and about 15 Euro teaching private classes.
I'm afraid I can't answer your questions regarding your art, as I have no experience with that.
Best of luck and let us know how you get on!