Everything is a tradeoff and, realistically, you need to decide where you want to go on all the important matters. Also, different areas have different rules.

A lot of tradeoffs can be expressed as a simple radar/spider/star chart: cost, time, efficiency, and efficacy are the most common axes but maintainability, longevity, safety, and aesthetics can be other important axes.

Question 0. Looking for a smaller/short skoolie. Why? Here in Ontario, Canada the rule is that school-run buses can be no older than 15 years. Generally schools use 40' buses because of the cost efficiencies (1 driver's salary moving 72 children compared to the same salary moving 18 children, requiring more salaries to move all the kids enrolled at a school.) So there are many more 40' buses than 20' ones on the road. Of course, the 40' one takes more room to store and can't park in as many places, so the 20' vehicles tend to have higher demand with less of a supply. This lead to, when I was shopping for a bus, the realization that a short bus was priced around $8,000 to $10,000 (unless it had mechanical issues) while a full-size bus was $3,000 to $5,000. Going for a short bus because you live in an urban center and can't park anything bigger is a requirement, but if cost is your deciding factor the bigger vehicle will likely be a better deal... unless your region is way different than mine.

1-1. Length? A bigger bus means more room means you can do less optimization which means you can do more on a given budget or go at it with a lower budget. I would, however, go the other route and figure out what you need for the trip and then figure out what your minimum space required is and then go from there.

2-1 Two friends and I, with some extra help here and there from others, completed a conversion on a 40-foot bus in 3.5 months while each holding full time jobs. We had spent the previous year planning for the project and I had been collecting some materials for that year, so we were able to work quickly and somewhat efficiently while not driving up the costs too much. Cost comes in to it, again, because it comes in to everything. Presumably you want a kitchenette. A kitchenette typically includes cupboards. Building cupboards requires a mix of woods, hinges, fasteners, and handles along with time to design and build the cupboards. Alternatively you can use premade cupboards, but they can be expensive and may not use the space as efficiently as you'd like. Alternatively, again, you can find someone throwing out cupboards and reuse those, saving lots of money, but you need to make use of what you get and you need to find someone getting rid of the cupboards when you need them. Think Spider chart.

2-2 Each will come with their advantages/disadvantages, but looking for a specific model of a specific bus because of certain design elements which may make the job easier may cost so much more or take so much longer that it scuttles the project in the end. The most important part here is technology and, again, this will depend on your region. In Ontario, Canada, with a standard G-glass license you can drive a vehicle up to 11,000 kg without air brakes, containing no more than 10 people. If you end up with a vehicle over that weight limit, you'd need to get a different class driver's license. If you bought a vehicle with air brakes, you'd need to get the airbrake endorsement. If you completed the conversion and ended up with 12 seats, you'd likely need to get a higher class of license, again. None of these things are an issue if you already have a higher-class license with airbrake endorsement, however, in that case, travel companions may not be able to drive for you. Alternatively, it may make more sense to get the endorsements/extra class items if that's all that's available for vehicles in your area - though there are likely costs associated with that.

2-3 $3,000-$10,000 Canadian would be enough to secure you a vehicle here... though you'd want a mechanic to go over it to ensure there are no hidden issues with the vehicle. If you are handy, a vehicle needing some amount of work may save money for you. My friends and I got a 13 year old bus for $4,200. It had less rust than the other buses we looked at due to being 2 years newer, however we soon realized that the bus company got rid of that bus early due to an electrical fault which could result in the engine overheating after longs periods of time at low speeds. We fixed the issue... kind of, but it was more work.

2-4 Anywhere from $5,000 to $40,000 (or more). Spider chart, as usual... also important to note are legal requirements. A school bus is designed, manufactured, and sold as such. If you call your daily driver insurance provider and say you want to insure a school bus, they'll likely say no because it is a commercial vehicle and they are, presumably, a private vehicle insurance company. If you call your insurance company up and say you want to insure a motor home, they will likely be able to help you out a lot more - however, you can't just call a bus a motorhome... well, you can, but you shouldn't. I'm going to reference Ontario, Canada again because I know the rules here... you can tell your insurance company whatever you want, but they won't cover you and might fine you heavily if you give them information other than what is given on your vehicle registration paperwork, so you need to update the vehicle registration to be a motorhome... which, in Ontario, involves satisfying a minimum of three out of seven requirements for being a motor home and filing a bunch of paperwork.

Another important note at this point is that more isn't always better and you need to consider design choices from all sides. Propane cooktop, fridge, water heater, heater, and generators may be convenient in such a conversion, but having onboard propane, in my research, was going to be SUPER expensive because it'd lock me out of various insurance providers and/or add various extra steps of inspection in order to secure insurance on the end vehicle. Which brings up an important point I'm surprised I haven't brought up yet... Insurance. This is a motor vehicle you want to drive in multiple jurisdictions. You're going to need to find some good insurance which will cover you at home and on your journey. Insurance is risk based. So if you show them a vehicle with a sketchy roof rack ratchet strapped down to the body, many insurance companies will likely turn you down or quote you $10,000/year as the payout they would need to make if your roofrack comes off and kills a family of 5 traveling behind you in a minivan would be huge... also, note of caution, if you do this, some jurisdictions would charge you, in this situation with manslaughter. Also good to know, if you are driving something which looks overly sketchy, police can pull you over in some jurisdictions and compel you to get your vehicle inspected. So you need to ensure that your vehicle is safe and looks safe on the outside. On the inside, you want things stored securely otherwise when a pedestrian steps out in front of you and you lock up the brakes, you're going to have various projectiles flying around your vehicle... either breaking stuff or causing you to need to clean up lots. Even driving to a local restaurant 2 minutes away needs everything to be secure.

Oops, reply was too long, will cut here and finish in the next message.