I recommend that thin pink plastic sheeting that you sometimes see in packing containers. My motherboard had a sheet of this underneath it to protect it. This looks to be completely water and mold proof and very dense. A roll of this is much cheaper than a Dynamat.
IME, the biggest area to take care of is the roof - mat under the headliner is a must. Secodn of course is the firewall. I'd be tempted to weld in a plate if it was a frame-off project and fill it with sound deadening foam(kind of like making a double-pane glass window). The floor is simple enough - a sheet of whatever and some thickr carpet works well(should be about twice as thick as stock(which was budget junk to begin with). About as thick in total as a factory floor mat or the rear cargo mats.
