Foam and Deck Replacement

O.K guys, I'm looking into ordering the 2lb density foam, and getting some plywood for the deck. My question is how do you know how much foam you need and how much to pour into the cavitiies? Also can I just use exterior grade plywood, and what thickness should be used? Another thing is what do you seal the gaps in the plywood with (reg. caulk)? I have no idea how to go about installing the new deck, and I really don't want to have to remove the foam again, so all your help and guidance is really appreciated. Also one last thing, the main stringers seem fine but the outer ones and the cross supports are kind of shot. What type of wood do you replace them with and do they need to be glassed in? Thanks

Is there anyone in CT that would be interested in completing the job? For a price of course.