Tim, the Cat 3 requires protection for the crew in poor weather, ie a cabin, although some seem to get away with drysuits and a restriction on the Cat 3. However suggest you look at Cat 4 (how often are you going out a night time?). The codes are about to be harmonised so print off this and have a good read, although its a huge document.
You will need to have a stability test carried out with the surveyor, so before going ahead carry out the test yourself with some friends to see if she will pass.
The rules for the local harbour coding have changed and you now need a liferaft. The costs over 5 years are now similar to the MCA coding therefore you may as well go down the MCA route and be able to use the boat country wide rather than be restricted to the Solent.
Duncan (Searider on ribnet) is a mine of useful advice and carrys out my surveys.