I didn't speak Spanish before we moved there, but was fluent within a year. It's actually pretty easy to learn - most of mine was done by listening to Spanish radio in the car and watching the Spanish news/reading newspapers and then testing it out...
The international schools cost around ?3400-3800 for your age children (per term). The Spanish schools near to where we were are full of English children...The one down the road from me has at least 26 nationalities there and the lessons are conducted in both Spanish and English (depending on the subject). Non-Spanish speaking children that start at these schools are generally near fluent within 3 months, so that won't be an issue at all.
The nails course is a great idea - there are no end of salons up and down the coast, and I know/know of lots of women who do the nails at their homes as well. The prices there are higher than here too.. I think a full set of acrylic nails is between ?40 and ?60, so scope there. Also, there are always jobs going in bars/restaurants through the year and, of course, the many beach clubs up and down the coast, from Elviria - Sotogrande rather than the other way.
I would be cautious of plonking yourself bang in the middle of no one but British expats to be honest. They are hugely insular, the work that goes around is between themselves and it seems (to me) to be quite a hard circle to remove oneself from to venture further. You would be much better off looking in places like San Pedro/Cancelada (west of Marbella) and Elviria, Santa Clara, Ricmar (east of Marbella). You have access to some of the best Spanish (but lots of English speaking) primary and secondary schools in the Malaga region.
With regard to your DH's HGV - I'm afraid I know jack shit about that side of things, but I can't see why he couldn't perhaps work with one of the bigger English firms down there doing that. Decorating work wise, one can expect to pay about ?950-1300 to have your 2/3 bed apartment repainted, so it's up to him what he makes of it. It's certainly worth it IMO and there is work around - I would suggest getting to know smaller property management people (the ones that work on their own mostly) and join up with them to boost contacts.
I actually ended up with my own business, started by simply looking after a couple of places for friends who only came over during holidays and the odd weekend. One minute I had two to look after, 7 months later I was running 35 properties
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We left, well, personal reasons mostly, I miss bits, but I am glad to be back. Very glad! However, we will be to and fro anyway, but now we are enjoying being back in London...