What about a property in the Sounds?
Say so in the request. Water access changes both who can take the job and what it costs, and it is far better in the quote than discovered on the morning.
How is it priced?
Usually a rate for a set number of hours, plus travel and sometimes gear hire. Ask what happens if the night runs long, since overtime is charged and is easier to agree beforehand.
Can I choose the music?
Most will take a must-play and a do-not-play list and read the room in between, which is the part you are paying for. A DJ who will only play your list is not really being used.
Do they bring the sound system?
Usually, though some venues have their own and some have sound limits or curfews. Say the venue in your request, because it changes what is needed and what is allowed.
How far ahead should I book?
For a wedding or anything in December, months. Weekend dates go first, so give the date early and you will have more to choose from.