Oooh... we do love a good build thread!!
Your theory sounds good... ...not sure how cheap it would be to do. Assuming you are using genuine Marine Ply you would need at least 2 sheets for the base, plus some frames plus a deck I guess? Or just stitch and glue?
Must be looking at £150 for the plywood delivered plus £50 for epoxy plus £50-100 for GRP sheathing. Then some form of coating and sanding - £50-100
Then you need a plan! You are constrained by the existing design and your easiest plan would be to copy the existing inflatable hull shape although something with a chine might be better....
Oh and you'll still need a transom!
Back of a fag packet £500? But your boat will be heavier. Do you have a trailer. Is your engine big enough?
Plus you need to glue the tubes to your new hull Thats a fair bit of glue and needs to be done well.
Tiller steer or console?