Do your research. Take a look on paper and in real life at lots of boats. List the things you need and want (two separate columns). Start narrowing down the boats by using your list. Arrange to sail a few or talk to the owners. Don't be afraid to ask the hard questions... What do you like least about your boat, what problems have you had... etc.
Finally after you have your list down to a handful or less, do a comparison, eliminate the similarities, and identify the differences that are important for you. You should now be down to a couple or three boats. Do your sea trials, hire a surveyor, then make your offer.