I went through the same situation as you in February this year. I wanted a boat big enough to go offshore in good conditions, also something with a cabin for the kids to get out of the sun during the summer months. I ended up with a 5.5m cruisecraft fibreglass with 135hp outboard. IMO 5.5 is about the minimum you need if you want to go offshore. Just make sure you get something that's got a good trailer and a motor with reasonably low hours. I'd recommend a fibreglass boat as they have a softer ride for the Mrs and kids. Good luck.