Sounds like a bigger job all of a sudden.
The floor needs to be sealed and any lumber will not do the job. IF You want to do it for the last time, you need a rubber membrane roof material over the new deck you build. Basically a new floor with plywood, no PT needed. Then the rubber, then a floor material which floats on top of some boards lying in the opposite direction to attach to your new decking...got all that?
Have a contractor stop by and give you some advice and prices, ask a ton of questions, and if it seems DIY, then slip him $50 bucks and ask if he''ll be your new buddy on this consulting project.
Big job = yes, DIY = yes..... Do you have 3 weeks to park yourself on it = ????
Or we will be around too.