You're talking about a lot of work there, what kind of area do you have to do all this in? Covered garage? Back yard? Time to finish is dependant on your level of experience & if you have to work outside, not piss off the neighbors, etc. For everything you mentioned a couple months would probably go by real quick.
Foam core deck materials are nice when done right, it's a little more work and you need to plan for any future installs to the deck (seats, leaning post, etc., can't put a screw into it without a backing plate) but on a boat that size you probably won't have that issue. You also won't save that much weight by just doing the deck in foam core on a boat that size, you'll save some but at a pretty good cost increase. Good plywood prepped right will last a really long time.
