welcome to ribnet!
The good thing about SIBS is they are nice & light, so no need to buy a huge trailer.
For the axle / suspension, you'll need to work out the weight of the boat, then add around 150KG for an unbraked trialer (unless you are towing with a Fiesta or similar, in which case check the car's max unbraked towing limit).
Not having towed a SIB myself, I'll leave it to the SIb owners to givce their thoughts, but there are plaenty of small ready to go trailers out there.
For what it;s woth, I looked at building my own for the rib, but when xcosted out, there was something like £50 in it by the time I had galvanised the frame & bought all the bits. That may be different for a lightweight unbraked one tho'.
As a thought, might also be worth looking at dinghy combi trailers.