Get a copy of Charles Kanter's 'Cruising Catamaran Communique' (his proof reading sucks, but the data is good) as a general guide to what makes a good and bad cruising catamaran.
Chris White's books on multihull's are also exceptional data sources and must reads for multi-hull'ers.
Next, subscribe to the UK's 'multihull' magazine.
If the budget permits, charter a cat before you buy it - what looks good on the dock or a shows doesn't always translate to works in life - try one before you buy it.
Also, time and budget permitting - don't just take sailing courses, take diesel repair, electrical repair, woodworking (the skills translate to fiberglass and steel sometimes) etc.. Living on a boat isn't like living in a house, the repairmen do not come to you, you are the repairman.
Stay away from bad designs, unsafe designs etc.
My wife and I went with the perfect for us 'couples' cat, you'll be looking for bigger. We have done weeklong charters on 44, 42, 41 and 38 ft cat's (going from big to small) working out what we did, and didn't like. During that time we had as many as 8 others on board during the week (four kids, plus thier partners). I could have handled a couple on the 44 ft, anything else gets my need for space and privacy on edge after a week.
There are a lot (LOT) of older cat's out there that make wonderful family boats, enjoy the hunt.