Re: How much hp do need?
Not necessary to use huge amounts of plating in there! Some 1/4 by 4 inch stainless plates on the bracket mounting bolts should spread the load nicely. Nice thing, with 1/4 thick plates, you can tap to the bolt size and with a couple of holes in the plate corners, bolt it to the inside of the transom. Now, you don't need someone inside to hold the nut. After you tighten all the bracket mounting screws from outside, you can go inside and use self locking nuts for safety. With the size engines you are contemplating I would use at least four 1/2 bolts on each engine side of a (single) twin engine bracket. If using two separate brackets, at least six bolts on each bracket.
Since your boat does not have a splashwell to reinforce the transom, I would also consider some knee braces from transom to stringers. Probably not necessary as the original I/O did not have them, but a little insurance could not hurt.
