You've given some good info., a bit more would be helpful.
How do you plan on using the bus. Full time with all your belongings, staying at campgrounds or boondocking / dry camping? Will you need space for adventure gear? What temperature zones will you be primarily in?
I personally want to keep moving, chasing the nice 75° weather. Do you want Gas or Diesel?
You can also look for a bus for sale, that have been started already by someone. Many times 'it's difficult to see such a big project through. I'm into my second month and still have a long ways to go.
You can also keep it really simple, remove the seats, bolt down the furniture you need and go.
Buses come in different styles, commonly flat nose, a conventional (dog nosed is what I call them) and the smaller van chassis type. There are pros and cons to each style. There are better names for these as well. I think the vans are class B's and the bigger ones are class C's. You'll have to do more research.
At 6-4 you may be looking for good head room. School buses come in 3 different heights. Fastest way to tell is by looking at the drives window compared to the first passenger window right behind it. It's the passenger window behind it is level with the top of the driver's window, it's a standard ceiling height. It goes up slightly between the two windows it's a high roof, if it drops down a bit it's a low roof.
Some people convert city transit buses because they have higher head room, but also tend to have really high mileage and not geared for highway speed. (Of course most school buses are not all that fast either) If you have the money, a commercial tour bus is a nice higher ceiling.
The year of manufacturing matters. 04 and earlier, don't have the emissions standards issues, 05-06 are middle years that the standards went up some, affecting both emissions and performance. 07 and on up met the full emissions standards, emissions went down and so did performance and maintenance costs went up.
These are just a few of the things to consider, plus there are exceptions and other details that may or may not be important to you.
You should also look into roof raises as an option for head room.
I'm sure I've forgotten to mention some other things. It's late and I'm ready to sleep. Just keep researching. One good debate is on what is the best diesel engine to have.
There are a few that are known as the most problem ones, but everybody has there own personal opinions.