Full time living aboard
As you no doubt suspect, there are a hundred (thousand!) different answers to your question. Everyone will have their opinion of what suits them, but it''s of course only your and your crew''s opinion that matters. Here are some guidelines & suggestions that MAY apply; hopefully, they are somewhat independent of personal tastes:
1. Think resale. Ex-Navy or not, there are many variables that will determine how happy all of you are living aboard and, even after a year or two of reasonable happiness, it may be time to move ashore. Be clear when researching specific boats of interest about how much in demand they are, generally. When/if you want to sell your boat, you''ll want it to happen easily, not with much hassle.
2. Use one or possibly two brokers. Many think this is a waste of money (the commission is the seller''s responsibility but the common view is that this inflates the price accordingly) but, since you''re entering into a new category of boat, an experienced, ethical broker can be a huge help. Do NOT work with a broker representing some of the boats you are interested in; s/he already has a client in the sellers. DO shop around - a lot! - to gain word-of-mouth about brokers who tend to specialize in the kinds of boats you are interested in (U.S. made? Taiwan built? Locally (Oz/NZ) built? Glass hulls? Reps cruising boats vs. racing or charter boats?). Good criteria to start with are 10+ years of a steady brokering career, preferably all of it with one long-term brokerage that has a solid reputation. Normally, you would work with only one broker (an ethical choice on your part) but perhaps Sydney''s boating areas are spread out enough that a broker would not be able to knowledgeably represent and show boats thru-out your area; discuss this with any potential brokers when you *interview* them (before deciding on one). Again, the right one can bring much to the table IMO but you need to select him/her thoughtfully. This takes time that may seem stolen from boat shopping; it''s time that will pay dividends later.
3. Mooch/beg/"drop in" and otherwise visit boats *as a family* as much as you can. Notice how each member of the crew reacts to each boat''s features - it''s the features that you''re interested in at this point, not the whole boat. For some, lots of light is essential. Some favor center cockpit boats strongly, tho'' I think that a non-starter in your size range. Where does the head need to be when living aboard (imagine having a guest visit)? Where does it need to be when offshore and the boat''s heeled over and bouncing around? How''s the galley look; what kind of feel does it have for the cook? What features lead you to that reaction? This is lots of fun, and it builds buy-in from the whole crew to the general idea, too.
4. Preferably, every member of the crew should have his/her own private space that s/he can retreat to, "close off", and be alone in. On our second boat, our family of 3 could do this and the boat was only 20'' long - don''t underestimate the importance of this to your son, especially as he grows a bit older. ''Private'' doesn''t mean totally closed off and, in fact, 30-40'' boats with lots of tiny cabins make the whole boat feel claustrophobic and smaller than it is. ''Private'' means a space all your own, where you can leave a mess without someone else having to deal with it, and which is visually separate (curtain, partial bulkhead, door, whatever.
5. There''s a natural bias being introduced into your shopping when you include the criterion that you might want to take the boat into blue water. E.g. it''s believed offshore boats should have small cockpits. Perhaps so (tho'' some more so than others...) but for living aboard, a spacious cockpit can be a joy. E.g. where does your son get to have his mates visit and play with him? Usually when on the boat, it''ll be in the cockpit unless weather says otherwise. You''re trying to mix a tough blend, offshore qualities with living aboard while working, not cruising. Bend in favor of the ''living aboard'' criteria, as that''s more certain than the some-day blue water adventure.
I''m sorry there''s no lengthy list of specific brands and models, but I think it more important you form those views as a family. Besides, here in the States we''ll have a fairly inadequate view of what the inventory is like in Sydney, NSW. But good luck on the adventure, which is very much what it is. We''ve lived aboard on 3 boats as a family and, despite the many challenges, I would hate for any of those years to be taken from us.