300 photos sounds about right to create a holiday photo book. I have made half a dozen books through a company here in the UK and have been thrilled with the results. My advice is:
don't make your pages too cluttered
stick to a neutral and limited palette when choosing papers embellishments etc
don't be afraid to use not such good photos or parts of photos to tell your story. Your best photos can always have a page to themselves
be consistent with font size, shadow types etc
ensure there is a wide margin all the way around the pages you create to ensure that nothing is lost if the page "slips" in printing.
check everything thoroughly before you send it off to print - typos and silly mistakes seem twice as bad when fixed for all time in print