Re: first time buyer,
3/4" marine grade 7 ply, stainless screws. No need for coating it. Marine grade adhesive for your vinyl. Tried and true combination and will last a long time. I would save the $$$ on some of those other re-deck approaches and spend the $$ on topside stuff you can use!
Since you are re-decking, "you might as well" check into underskinning your pontoon with aluminum sheets and see if that makes sense to you. Many threads on here about it. It is a $400ish job. The benefit is speed performance (a 3-10MPH gain), less water rushing noise, and a less turbulent stern wake for your motor to bite into.
IMO, for your application, I would not underskin. Your motor produces enough HP and you'll go fast enough, water rushing noise at high speeds is part of boating, and stern wake is so low on the list of priorities!