Welcome. I'm fairly new here as well but I've read a ton already and learned a ton already. I'm working on my bus build as well currently, so I can give you advice up to the point where I've reached, so feel free to reach out to me.
So far here's the order of operations I'd recommend before putting a ton of money into a full Bus build.
1.) Ensure rust is at a minimum or mostly rust free, or can be fixed by replacing with fresh sheet metal. A rusty bus will always continue to rust. This can also be determined by how many years you wish to own said skoolie. All buses come with a little rust, and can be remedied if small amount. Treat with POR 15.
2.) Ensure engine is mechanically sound. Don't take it to just anybody, search for someone who knows Buses well. Some mechanics claim Bus and Coach, but they don't know older Buses. Find someone and call them and find out if they are an expert in your model. You may have to drive a good distance to find someone capable so expect that. Many people build a whole Bus and then start to drive it and realize the engine is shot. It's a 10k investment easily to replace an engine, if not more if a transmission has to go along with the new engine.
3.) Ensure suspension is fine. This includes king pin checks, (I'm gong through this now as I have bad king pins and it's a $4500 fix if you pay a mechanic to do it, about $1000 if you are capable yourself for parts etc.). I determined my king pins were shot because the steering tire was worn on one side, if you can rock the wheel back and forth more than 3/4 of an inch, they need to be changed. Shocks, you are going to be adding weight to the bus so shocks need to be in order. A shock set can run you $400 if you go to the right people.
If all of the above are in order, or acceptable for you to correct, then proceed. Otherwise, find a new bus that better meet the criteria above. The rest of the problems with a bus build will be easily addressed by you as you build the cab up.