Ok, well I would say that a lot of the tourist trade in Cadiz will be with Spaniards - the Spanish do a lot of internal tourism. That's not to say you wont get other visitors of course, although Cadiz is very popular with Spaniards (more so than, say, Marbella or Ibiza) as a holiday destination.
What else. Well there's a fabulous Catch 22 situation regarding buying property here - you need a NIE (ID number) in order to buy property. This is relatively easy to get - you make an appointment at a police station - except you need an address beforehand. So you'll probably have to move over here and rent short-term to sort out the administrative issues before you buy. You also wont be able to get a resident's Bank Account without an NIE either. We bank with Lloyds Espana which permits free transfers between the UK and Spain, though I don't think there are many Lloyds branches here.
You'll also need a Social Security number in order to pay tax, which again is very straightforward, and an Empadronmiento (Certficate of Residence) which you need before you can register with the health service or to vote.
If this sounds daunting don't let it put you off - it's a wonderful country and the people are very welcoming and friendly, especially you if make an effort to get involved.
Also be warned that the Cadiz accent is reputed to be the hardest to understand in the whole of Spain!
How does your 12 year old feel about the idea of moving?