£500 is a good start off point, obviously explain to the couple that you are charging this amount (low amount) because you are new, and i am sure they know how much an average tog charge if they have done their research.
10 days is very short, you can give yourself 28 days in the contract, however that will just tide you down a deadline and if you fail, they can claim for all the monies back. Personally I wouldn't put in a date in the contract, if they ask, then say you try provide them with the proofs within 28 days, but also tell them that your own timetable is usually shorter etc. Then after they get the proofs (low res, good for screen but not print) then let them pick the photos for the book. Personally I would also pick my favourite 150 or so, and ask them if those are okay and feel free to amend my selection. It's so that it speeds things up.