Average pay is 10 to 14 pounds an hour, but a teacher will only usually do (and be paid for) 24 hours a week, whereas people on the minimum wage could easily do 40 or 50 hours and so be paid more. Sorry, but that's just the way it is. There are 100s of language schools, but there are 1000s or 10s of thousands of TEFLers going back home with no other job skills.
Yes, DELTA does make a big difference. You may be able to get on a DELTA course without doing the CELTA first, it depends on how much teacher development (observations etc) you have had, how well you know basic teaching techniques and jargon (PPP, TTT etc), and how well you know phonemic script and grammar. Phone up or email a local or online DELTA provider and see if they think it is possible for you. If not, do the CELTA or Trinity CertTESOL. You will not be able to find a job in a decent school without one.
The point with summer schools is to choose a school where you will be working in or near the main, year-round centre. If the permanent teachers decide to leave in September and you made a good impression, you will be first on the list.