I think other people will have more insight but you should figure out what your priorities are first.
Most if not all of these buses have some rust on the chassis. Some leave it and never have issues.
To possibly save you research time, I treated mine with Ospho and POR-15 ($50/pint). Both of these (in my research) are the best possible products for the job (just be sure to degrease before and after Ospho), however, you may find Rust-Oleum Rust Converter ($6 spray cans) and Rust-Oleum enamel sufficient.
You'll want to budget more for insulation. Foam insulation, sheep's wool insulation, pink/green insulation panels... Whatever you choose you'll need to keep moisture in mind. Moisture will eventually lead to mold. Spray foam and (real) wool insulation will handle moisture fine. Moisture control in the insulation stage, proper painting precautions and not storing wet things in dark places will go a long way.
I've seen a lot of people go all out on insulation but skip gutting the floor and the roof. It's my opinion that it's a waste not to insulate/reinsulate here. It's more work but these are two massive pieces of metal conducting heat into/away from the living space. Please look into "thermal bridging."
For better or worse, Wal-Mart is going to be the cheapest option for new spray paints and gallon paints.
Lowe's and Home Depot both sell mis-tinted paint at a steep discount. Generally, $9/gallon. I have purchased Lowes's top tier Showcase paint several times for $5/pint, $9/gallon, and $5/half spilled gallon.
Black mold can develop. I recommend budgeting for Killz primer (alternatively, waterproof deck stain + sealer) on anything wood.
For the roof, it's generally recommended to use a white paint. Some people swear by Tropicool. I used Bus Kote. There's some thought floating around that mixing a medium such as fine sawdust in with white paint will be effective as well. Some people just opt for a standard paint in white. You do not have to paint your roof white but keep in mind that this will have a direct bearing on interior temperature.
Buses are made to carry large amounts of weight, however, each bus has their limit. Look into the gross vehicle weight rating (GVWR) of your vehicle. If you have a short bus like I do, you will have one for the van chassis when it rolls off the line from Ford, GMC, Chevy, etc. and a separate GVWR after Thomas, Bluebird, etc. has modified it. My knowledge here is very limited but modifications can include suspension changes and brake size/effectiveness changes to better accommodate a bus's typical use. The GVWR after (bus) factory modification is the one to go by.
It is recommended to build a bus so that--when fully packed with furniture, food, knick knacks and doodads--your bus's weight is lower than this number. If you are exceeding this number you will be stressing the suspension, braking systems, maybe even engine. It's not the end of the world if you exceed this number after the build is finished but just keep this in mind while you're in the budgeting/planning stage because it will affect which materials you use, which brands you buy, and where costs are to be cut.
You're going to need a larger solar power bank/generator/shore power. Your budget will vary wildly depending on these options.
If you go solar, consult Will Prowse's YouTube channel. This was the single best resource I found (and it's all thanks to some random guy on Facebook Marketplace, ha).
Cassette toilet? Composting toilet? Flushing toilet with black water tank? Incinerating toilet?
Fresh water storage? Grey water storage?
Electric pump? Foot pump?
Custom may work better for your space but will take time + money. Putting a big ol' honkin' queen size bed in the back is a common choice.
Again, custom fixtures, more storage, less storage.
Traveling with another human? Seat belts, larger/additional sleep space.
Traveling with an animal companion? Separate food storage, sleep space, always on A/C, etc.
Advice I've seen is to have enough to cover an engine/transmission replacement just in case. Also, depending on bus size, tires can be expensive.
Tips:
Reuse your current mattress.
Upcycle!
pallet wood is demanding a premium but there are still places that throw them out. I got mine from a flooring store that was begging people to take them and they get more each week.
Habitat for Humanity can have some good deals but they also have some ludicrously priced things.
Thrift stores.
Closeout stores. I'm not sure what location/chain is operating near you. For me, it's Mike's Merchandise. A lot of these stores receive Amazon returns and more frequently than you might think, some of these are unopened or opened and immediately returned. I purchased a bar sink for $80 at a time when Amazon had it listed for $180 and there wasn't a scratch, crack or any other blemish. If you're in the Birmingham, AL area you should stop by Mike's Merchandise once or twice a week.
Amazon returns. Amazon Warehouse deals can be marginally decent. They can also be really good. One YouTuber said they were able to purchase a collapsible ladder for $50 (typically $150+).
Curbside the night before trash day. For real though. I've been doing this since I was a kid. Got two big screen rear projection TV's, 2 Razor pocket bikes, plywood for my build (one sheet turned into my dinette table), a bunch of things.
I used Pinterest to quickly find ideas that I liked and was able to quickly peruse when it came time to actually build.
Check out YouTube build videos, build blogs, etc. Different perspectives can really help you in the planning stage.
Be ready for things to not work and understand that improvisation may require a little bit more money than intended. Even with all the pallet wood I've used I have had to purchase additional wood here and there.
SCREWS. Save the screws from your bus panels to use later in your build otherwise you're going to spend an absurd amount of money on screws, nuts, bolts, fasteners, etc. I had two large bags of screws. I'm mostly through my build now and still have half a bag left.
Support system. I don't know what your personal life is like but if you have family and/or friends that support you it'll go a long way in the build and towards your mental health. If you don't have that support system I recommend befriending people in the community.
Skoolie.net seems to be the go-to forum. I became a member really late in my build (6 months in) so it hasn't been uber helpful to me but it's a plentiful resource and if I was in the budgeting/planning stage, I'd do some lurking there before making a move.
Use a rubber wheel to remove stickers (not an affiliate link).
Lowes has overwhelmingly better prices on "specialty" fasteners (corner brackets and braces come to mind).
Butyl tape is cheap, waterproof, and I've legitimately used it in almost every stage of my build process. At <$7 I purchased two rolls for my 20-foot short bus and still have some of both rolls left. Best waterproofing (and sound dampening) product I've ever used. "RV repair tape" is butyl and fabric mesh at a marked up price.
If you plan on running your vehicle overnight or using a generator close to the vehicle, a carbon monoxide alarm is cheap assurance that you'll make it to the next day.
Have a fire extinguisher. Preferably one in the cab and one in the kitchen.
Non-skoolie but related things to follow:
This guy does unpaid head-to-head product comparison tests. He's a retired vet and refuses paid sponsorships so you're getting the most unbiased content out there.
I have purchased and used some products (Seafoam, POR-15) specifically because of the test results on his channel.
This is Will Prowse's website (same dude from the YouTube channel I linked earlier).
Edit 1: I'll update as I think of more.
Edit 2: I added a bit more and swapped segments around in my personal perceived order of importance.