First to consider.
Its a short boat, any steel you add will drastically affect the draught and the trim so you will have to ballast it when it is afloat and you have added most of the fitout weight, stove, cooker, mattress, etc. Leave floor panels removable, not trapped under bulkheads etc. so you can add ballast where it is needed. Scrap steel takes less room than bricks, sash weights, steel billet offcuts etc. and don't soak up water to cause more rusting. Remember filling diesel and water tanks will seriously alter the draught and trim.
Sounds like a very old Springer with a timber top, you are far braver than I to take this on. It was probably built with 4mm or 3/16" plate, take care, insurance companies are refusing to insure if any part of the hull is below 4mm thick.
Secondly,
The timber top is an annual repair job, it will leak where it joins the steel hull, always. And round every window and door.
Be warned, not discouraged, but this could be a real money pit, both now to get it back in use and forever going forward. What engine/gearbox do you have and what condition is it in?