Depending on how "all out" you go, you can easily eat up eighty to a hundred and twenty sq. ft. of material.
The trunk alone is around 40 sq. ft.
With all that said, I would personally go with a shop pack (which is eighty sq. ft). This will give you plenty of material to work with.
As for the spray on bed liner, you would want to apply it over the mat.
In all honesty though, bed liner doesn't do much for sound deadening (it's not elastomeric), but it is good as a protectant if that is what you are wanting to achieve.
If you want protection PLUS sound deadening, there are many spray on sound deadeners out there that will work awesomely. Second Skin Spectrum, Cascade Audio VB-1X, and Lizard Skin just to name a few.
Later,
This!!!