I was going to suggest that your best bet is to join tghe club, come along to a meeting and get to crew on others boats. This way you will be able to see the differences between boats.
Re: Fishing 4 people, I have a 17 ft boat and have fished with three ( it is rated up to five ! ) but even with three it gets cramped ( actually can get cramped with two when Sam comes onboard with all his sandwiches ).
ie. You will need bigger to fish 4.
I am considering going up in size to a 19ft but the costs rise exponentially.
re: Trailing or mooring. I would say trailing is much cheaper.