I think that's a pretty intersting idea. However, like any other decision, the answer to whether it would make a good BOV is, it depends.

Off the top of my head, I can see quite a few pros and cons

Pros -
Large capacity
Heavy (if your dream of ramming other vehicles becomes reality, more weight is better)
Non-threatening looking
Good visibility (for passengers)

Cons
Heavy
Poor manuverability
Poor gas milage
Too open (no privacy for passengers
Top heavy, if in high winds
low ground clearance

Look at it from a design perpective, a school bus has been optimized for one thing, moving large numbers of passengers in relative comfort, over well maintained roads for short distances. I actually think in that respect, it might be a really good BOV, that would satisfy most peoples needs in reality. Though most people prefer to think that their needs would require them to blaze a trail in a large 4x4 to some mountain retreat, it seems a majority of people just sit on a highway in traffic. Although I do think ground cleareance could be an issue in this type of vehicle. I have taken a lot of regular cars on unexpected "off-highway" excursions, and usually ground clearance and approach angles will stop you long before lack of traction will. There are always chains to improve traction, but nothing you can do if you're high-centered on an obstacle. As for the dual wheels, that could be a plus or minus. Single wheels might be better in situations where a tire can dig into a layer with better traction, but if in sand or deep snow, dual wheels will allow you to "float" on top of it without sinking in.

From your description, you mention using it as a semi-long term living quarters. As other people already mentioned, there vehicles that have already been specifically designed for that purpose, and have undergone many iterations and millions of man-hours of road testing to optimize it as such, so it doesn't make sense to re-invent the wheel.

But rather than scrapping the idea, you have to think of where this vehicle might become useful. If you have a very, very large family, or group of friends, then this could become very useful. As far as using it to move things, I see one major drawback, access to the inside is limited. A small moving van (like u-haul or something) converted with a couple rows of seats might be easier than trying to add doors to this.

As far as find a usable bus in decent condition, I really don't think that's much of a problem. I assume most buses or government vehicles like this would have a regular mechanics that constantly maintain it, so you are just as likely to find a decent bus as you are any other vehicle or similar age. It might even be better, because you can almost assume it's lived a relatively docile life, no street racing, no off-roading, etc. Of course, if you buy a $500 bus, you can expect to get a $500 vehicle, doesn't matter whether it's a bus, a car, or a truck.