A lot of people have chimed in to steer you away from your original goal - a converted school bus. Note that none of them own a school bus, and that none of them can give you a very good reason for their bias. They even admit it.
I own both. I currently have a 36' flat nose Blue Bird school bus powered by a Cummins diesel and a 14' GMC box truck, also powered by a Cummins diesel. To date, I've actually owned four box trucks and three school buses.
Having actual experience with both types of vehicles, I'll take a stab at a more fact-based comparison:
School buses were the direction you were originally leaning, and they have quite a few pluses. First, purchase cost will likely be significantly lower for a school bus than for a decent box truck, and you will end up with much more space. School buses are far safer, both because of their stouter construction and because any collision will likely occur below the floor line of the bus. This margin of safety is by design.
Parts are not the problem others have claimed. School buses are built on medium duty truck chassis. Medium duty trucks are everywhere, so new and used mechanical parts are equally common. Every half decent sized town I've ever been in had a NAPA and at least one junkyard devoted to medium and heavy trucks, and that's all you'll need. Parts are generally more expensive, but then medium duty trucks are very durable - they're designed to be heavily loaded and then driven by indifferent if not abusive employees.
Since you'll have more space to finish, you will likely spend a fair amount more converting a skoolie. You don't have to, but you're doing this for your family to be comfortable in, so I suspect you will. There is also the matter of removing and skinning over many of the school bus windows, which you will likely want to do both for privacy and improved temperature control. That adds to the cost as well.
Having said all that, with a growing family and a dog, I suspect the result you'd get with a bus would be well worth the extra money and work.
You'll spend a lot more on tires for a bus, because they're larger. You can buy a good quality 16" LT truck tire for a box van for $200, but a good 11r22.5 for a bus will easily set you back $500. (On any vehicle, tires should be replaced when they're 6 years old, even if they have plenty of tread left on them.)
A box truck has some very different advantages. Box trucks tend to have pretty high ceilings compared to buses. (Of course, many people raise their bus roof a foot or two, and the result is fabulous.) Also, those nice vertical box truck walls are really easy to finish. Quick, too. You could certainly have a livable space finished in less time with a box truck than if you were converting a bus.
A box truck should also get somewhat better fuel economy, unless you're towing something, and a box truck will probably cost less to operate and maintain.
I say probably, because I know I'll spend more on tires and fuel for my skoolie than I would for my box truck. Then again, I can get a used or rebuilt Cummins pretty cheaply if I ever need it, and my skoolie's Allison Mt-643 transmission costs significantly less to rebuild than the modern electronic overdrive transmissions in any of the newer box trucks. Also, I spent so much less to buy my skoolie that I can pay for a set of new tires and at least one major repair on the bus with my initial savings.
If you plan on solar power, a box truck's roof is big enough to accommodate 750-1000 watts of PV. A skoolie roof will take twice that, with room left over for a deck. A box truck will struggle with the weight of the batteries for such a system, whereas a skoolie won't notice the weight. (This is one of the areas where a skoolie just can't be beat - when I started my conversion, I had 7.5 tons of usable capacity to work with. That's what a bigger box truck weighs, fully loaded!)
The downside of this size is reduced maneuverability. A box truck is easier to get around in, plus you have a lot more parking options. Of course, plenty of folks drive around in motorhomes that are as much as 9 feet longer than my bus, and I don't see any of them stranded on the street, unable to park.
If you were leaning toward a bus, I'd probably recommend a shorter one than mine - around 30' length, with a flat nose - those have a shorter wheelbase and thus are really easy to drive and maneuver.
For myself, I decided to go with the bus over the box truck. I like the space, and I want a large solar system. (I'm building an off-grid bus with an all-electric kitchen, including a serious espresso machine.) I also like having significant water and fuel capacity, which takes a lot of space and is not at all light.
On top of that, I wanted a good size fridge, a way to wash clothes and plenty of room for storage. A box truck would have ended up really cramped, and just couldn't handle the weight. Even if it could take the weight, there simply isn't room underneath a box truck to hang the battery box and tanks I want.
This has only scratched the surface, but I hope this helps you consider some options in a better light. Only you know what the end result needs to be for your family to be comfortable and happy. I would be able to share more specific recommendations if we had talked a bit. PM me if you're interested...
