I can probably help a little, as I've lived on the Costa Brava all my life. So, my best advice would be:

1. NEVER, ever underestimate just how expensive the cost of living is here. Many people come over thinking that Spain is a cheap option. It isn't. Sadly I've seen many families crack under the strain of a misguided, underfunded and under-informed move to cheap, sunny Spain.
2. On that note, it is not always sunny and winter can be absolutely freezing. Definitely recommend visiting the area you intend to live in at least once a season over the year, to get an idea.
3. LEARN THE LANGUAGE. You don't need it to go to the pub, but you will for everything else (doctor, dentist, jobs, school...). Especially if you have a young child.
4. You will probably find it extremely difficult to find legal, decently-paid work, especially at first. Have plenty of money stashed away, enough for ALL you first year's worth of expenses. You will have a better time of it if you are skilled, but even so it can be tough.
5. Don't burn your boats. My advice would be keep the property in the UK if you own one, you can always rent it out in order to cover mortgage/generate income.
6. Make sure you are covered for medical emergencies, there is nothing worse than being ill abroad and if you're not insured it can be cripplingly expensive.
7. Bear in mind Spain does not offer the UK's vast array of safety networks (for instance, in order to claim six months's of dole money, you need to have worked for at least a year, child allowance does not exist and nor does income support). You are very much on your own.
8. It's worth requesting form E-301 (I think from your Job Centre) as this lists your emplyment within UK which would help if you ever need to claim the dole here.
9. The rent you are thinking of is only just realistic for a long-term, decent apartment in the areas you would like. Think in term of 500-600 € and you'd be close to the mark. Then add bills.
10. If I haven't put you off - Best of Luck