If I was buying a rib I don't think I would bother with a hull survey unless it was very old or didn't seem right in some way. Once ashore, you shoul be able to check the hull for problems.
However, I would get an engineers report on the engine. If it's relatively modern, then an engineer shoul be able to hook it up to a laptop and get a full printout of hours, faults, usage, etc. If it's a non engine management model, then you either have to rely on its service record or get an engineer to physically check it.
We typically pay £49 for a computer printout and about the same for a physical check.
Hope that helps
You get what you settle for!