Cold press is the way to go, but I would leave it more than 24 hrs if you could (maybe 2-3 days). As far as the amount, you would prefer to have as little water as possible with as much coffee flavor extracted into as possible. A half gallon of )coffee-flavored) water would reduce your gravity by ~.006, keep that in mind. Have you thought about just aging the beer on coffee beans for a week or two?
perhaps a line of single hop IPA's - there's so many new hops out there!!!