First of all, welcome! You should find answers to any questions right here.
Have a browse through the boats for sale section on the main site and keep an eye on the RIBs for sale section of the forum, you might be able to find something suitable there.
Drysuits are more common than wetsuits, as they keep you dry. Under the drysuit, wear layers to keep you warm. You could wear hats if you wanted, as they would also provide thermal protection, or you could look into Gecko helmets for the impact and face protection too.
If you want to be carrying 3 or 4 people and only a small amount of kit, it could be worth looking for a boat with jockey seats for 4 people, much more comfortable than bench seats or sitting on the tubes. Again, keep an eye out for second hand RIBs that can offer what you want.