The weakest point of an old school bus is the floor, they are not designed to hold huge loads, though people have been known to use them for hauling cattle in with minimal addition being done, you need to reinforce the floor where you would stack very much weight, or at least be able to redistribute the weight over a larger enough area as not to have the "stack" fall through the floor......
Ive been around a couple of school bus conversion, one for basically transport of 4-H folks, and a couple others that were hunting camps [same as a BOV only easier to explain to the locals] One nice thing is you can put your own FULL SIZE propane stove in and can actually have room to cook for a mess of folks if you have to, and you have the room to can on it too [also nice to use in the late summer and early fall when it is hot inside the house and you want the canning heat left outside....more justification] I would add a 4-5 foot expanded metal back deck extension [inset from both sides just a wee bit for looks] to set the propane tank(s) and extra water barrel on, and make it easier to load and unload through the door, similar to the lift but stationary and not as spendy. also used to set food off if you have a group to feed without having them traipse thru the bus [whether that be 3 or more folks in your family to a whole group of a MAG]
if you get the right bus, you may already have a "basement" underneath makes it easier to run any plumbing you might want to do and keep it from the elements, be it propane line, or water for self contained running. with the invention of the flex water pipe it is easy to run water service and not to hard to keep it warm if you also run along side of a heating vent [yes i can understand you dont want this atm, but one day....] i would place the potable water tanks on the main floor, and the black water-grey water tanks under the frame with the clean out similar to any other RV system, the reason for me is i do hunt, and any water in the main compartment is sheilded from the cold that much more.... plus its nicer to reach the water pump if you have to work on it or replace it [been there]
think about a small box stove for burning wood for heat, but also add a blower to tie into your floor vents, warm feet equal "happy campers" and also keeps the supply lines from freezing up..... all it takes is a small squirrel cage blower hooked to a couple of pipes wrapped around the stove, not some huge monstrosity of a tin box that takes up space.... Aluminum bubble wrap will reflect heat off the walls and keep any wiring from getting excessively hot [shiny side out if you just use foil paper, which is not to code but it has been done by many folks in their log cabins and the stove set way to close to the wall and they have no problems... use common sense though]
If you purchase an old RV motorhome that is damaged you can get quite a few neat things like a roof air and a genset sometimes for near nothing and swap them over to the bus, my digging partner picked up an RV with rof damage from snow, for $400 and it had a 6.5 KW twin cyl. ONAN genset..... $4000.00 new for that alone! plus dual roof air, nice RV shower, plus some other stuff that was salvageable and he drove it 120 miles home while i followed [whole nother trip but we made it]
One friend who lives up at Fairbanks bought an old city bus that was already prepped for artic weather [6 inches of foam insulation in the floor, walls and cieling] and converted it to live in.... but he added a diesel stove though when it got to negative 70 F he was using 55 gallons of fuel every 3 days.... he found a lady professor that took him in for now.... and they run a dog shelter! [another story waiting to be told] and his bus i believe has been once again converted to run a 24-7 micro-broadcast radio station. He had one sitting in my yard and we put up about an 85 foot tower...... and the FCC knew where we were, but we were down at the time they flew around us checking the signal strength out..... we had blown an amp the week before, yet again a whole different story line to get into on communications and entertainment. But that bus was a 1964 Hayward with a pusher and the escape door was on the driver side near the rear wheels, which is where he had put his wood stove.... I used to have pictures, but they are locked on an old HD that crashed....
Add a awning off the side, very decent investment, gives you added space outside the rig to set up a table and store some gear that takes space inside..... if the unit has a basement, add "drawer rails" so you can get things out and not have to crawl inside to get them.... again experience and not really that much extra dollars in setting up in the beginning, though by the time you set it all up to livable, you will have outspent some lesser valued RV motor homes but you will have what you desire for less than you can have it built for.
On the roof, i would add a storage rack and platform that you could "sandbag" and have the ability to shoot off of similar in design of a tower only shorter, designed correctly it will also be a platform to hook up solar panels to for charging a battery bank, and communication towers later on...... forethought.
need more ideas??? you can also did a utility trailer in a similar manner but you need something to pull it with.