You asked three specific questions in your OP. To the first I would say absolutely yes, go with diesels if at all possible. We put around 250 hrs per year on ours and like to cruise at 20 knts, so the fuel efficiency really makes a big difference. But, at least as important to me is that diesel is much safer IMO. In our boat, or any 45 or 50 ft you will likely find the genrator runs a lot although it depends on where you are. But with full size refrig and ice maker, and ACs, and TV and sat dish, it runs a lot and I am much more confortable with a diesel generator. In the Bahams for example, power outages occur and you sometimes have to run the genset a few days at a time. Also, the big props and low speed torque make docking and low speed maneuvering much easier than with smaller gas props.
The question of diesel ownership and repair costs, however, I disagree with some of the earlier comments. While diesel rebuilds do cost way more than gas engines, the key to longevity is maintanence. The sea water system, the aftercoolers in particular, are crtiical to keep in perfect condition. Similar is to not overprop your engines, they must reach rated RPMs at load. I think it is also critical to run the engines on long cruises at 65% load or less and always strive for 30 to 35% of WOT fuel burn over a years run time. We put 250 hrs on our boat per year for three years in a row now and have only had standard repair items except for a preventative, but costly replacement set of aftercoolers. We have our props tuned annually, bottom kept clean, rated RPMs at WOT reached, routine annual oil changes, impellor changes, heat exchangers descaled every year or two, etc. and have had three seasons of reliable diesel engine performance on a boat that was 9 years old when we bought her.
For financing, this is agreat time to put some of the purchase price on financing with rates below 4% depending on the loan details and credit scores.
Thrusters are nice, and you could always have them installed after the fact although that could be expensive. I use our bow thruster from time to time when docking sometimes just to ease in when alongside docking in some wind or current conditions or to allow the person on the bow to throw lines on a pylon. But I test my bow thruster almost every time before docking to make sure it is working, and also cause it is good to use it regularly as all marine systems, especially mechanical ones, like regular use.
I also would comment on your boat choice. We moved from a 30 ft express cruiser to a 50 ft flybridge and love it. We spend many more nights per year on our boat. In fact I have noticed that at many marinas, the boats in the 45 to 60 ft range get the most use as far as hours spent on the boat, although not necessarily hours spent out on the water. So if you want to spend lots nights on the boat with two kids, the bigger the better if it fits in your budget. The other thing I have come to believe is that money spent up front on a high quality boat is money very well spent. On the higher quality boats, construction methods are better, everything last longer, and all of the many support systems are made from the highest quality system manufacturers and require far fewer repairs making boating much more enjoyable. There are few things in life that I have found quality to be more important than in boats due to their extreme corrosive environment, irregular use patterns,and the physical beating a boat takes out on the water.
