Based on my own experience and on others tales I can give my opinion...
1) With a TEFL\qualification, a little knowledge of Spanish and a bit of assertiveness you will be able to find work almost straight away
2) However you will find it very difficult to obtain regular reliable work initially. It seems to take a year or 2 before you can get a good contract.
3) You will find there are very slow parts of the year. YOu will need savings to survive these slow times.
In short my advice is- if you are carrying debts, forget it. You won't be able to service your debts on what you will earn.
If however you come over with some savings (2000 pounds or more)to tide you over bad times, and are prepared to rent a room (habitacion) as opposed to expecting to be able to afford a whole flat to yourself, you should be ok.
One useful tip (which I wish I had done myself). You can learn a new language yourself at one of many budget language schools in Madrid. A daily 1 and a half hour 'intensive' course can cost as little as 60 Euros for a month. Alternatively, if you were to pay more and take a computing course (you will need to already have a good knowledge of Spanish) you will then have an extra notch on your skillset.
Good luck in whatever you choose to do.
PS It seems 5 times easier to get work in Madrid than in Barcelona. Too many beach-seeker expats settling there I suppose and clogging up the market..
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An English Bookseller in Madrid