If your buying second hand Rib consider a rcd cat CE mark B for a offshore rib.
If you happy using near the coast then a rcd CE mark cat C rib will be fine.
If your buying a workboat for pleasure then please bear in mind that the stability regs for new offshore pleasure ribs Iso 12217 havnt come in for no reason in offshore boat design.
At present the insurance companies have a cross over and are not insisting on stability regs for offshore work boat ribs, but I am of the oppinion it is only a matter of time before it will come into force.
As it is the perfect reason to limit our range and therfore there responsibilities.
Good luck and it may be 5/10 yrs before this happens who knows?