Welcome to the Madhouse! --- You found the right place to get started. Read everything you can before diving in. Then...dive in.
And yes, "newer" buses, specifically any made from 2004 up are best avoided. The geniuses at the EPA forced everyone to install a bunch of electronic crap that was supposed to save the planet. Instead...it is forcing people who ordinarily buy buses every 8-10 years...to buy new ones about four times as often because none of the miracle electronics remains functional for more than 2 or 3 years.
Do the "Eco Math" on THAT brilliant thinking. But...do NOT invest in it. Even fleet operators cannot afford to maintain them once they go south. That is why you see lots of newer units with very low mileage showing up for sale at fairly low prices. It is because anyone with any savvy at all...doesn't want them at any price.
They are NOT the deals they appear to be.
All of which is making it harder and more expensive to find decent, older units that don't have said electronics. Same thing with just engines.
Example: I wanted to buy a Cummins 4BT motor a few years back. I could have bought a new, "state-of-the-art" electronically controlled Cummins engine for HALF what I wound up paying for an older model. Why? Because the older, "all mechanical" engines are that much simpler to work on and maintain, plus they can be easily modified to increase their performance (HP & torque with improved mpg's) where the "modern" computerized engines are much more complex and expensive to maintain.
So...keep reading...and keep your eyes open for something pre-2004.
Best of luck on the hunt!