Just be aware with ribs that because the tubes overhang, there is a huge chance that your 5m boat will in fact fit a boat trailer designed for a 4(ish) m boat. Ribs are usually measured from the tip of the bow to the point of the cones. Both stick out to the tune of half a metre, and so if you look at a rib on a trailer most will have huge long drawbars sticking out the front because they were sized for the LOA.
I found this nugget out when I was trying to fit a 5m boat into a 5.7m garage on a braked trailer!
Other thing to bear in mind is towing weight. Not sure what your car is, but I'm going to make a stab in the dark guess that if you park a 5m rib on an unbraked trailer you will be sailing remakably close to the absolute unbraked max weight of 750Kg. This number is likely to be less than that unless you drive something like a Discovery, so it's well worth doing you homework to make sure you won't need a braked trailer.
Humber were really helpful when I phoned them to find out the weight of mine. Other option is if you are passing a weighbride then (assuming it's plated) subtract the weight of your current trailer from the number.
If I have served to worry / confuse it's probably best to post a Q in the trailers section. It's just better to do the homework now than find out the hard way from the traffic constabulary as you get hauled in for a random check.....