Are you looking to teach in a Spanish state school or a language academy? If its a state school I wouldn't know too much about the qualifications but €700 per month will not leave a lot of money for anything after rent and daily living expenses have been paid for.
However, if you are simply looking to work in a language academy then I would imagine that being a qualified teacher will open up a lot of very good opportunities where a 30 hour week will pay approx €1700 per month, which is a pretty decent salary (in Barcelona anyway).
If the language academy is something you're interested in then I'd definitely recommend the CELTA qualification, it is expensive but there are so many EFL teachers that, in my opinion, this is the easiest way to have you CV considered, plus it really is very useful, especially if this is your first time teaching a language or teaching student is a foreign language.
As for Spanish, the more you have the better for you to understand what the students are trying to say, however, in the academy I work in it is pretty much forbidden to speak Spanish. If you can learn the basics you'll be amazed at how much you can understand when listening, although speaking is much more difficult!
If you have more questions about language academies feel free to ask. Good luck.