In the coach world most older buses used high end stainless steel for the lower panels and suspension parts. It was highly resistant to rust and would hold up over a long period of time.
Greyhound would rarely let go of a coach that had less than 5 million miles. The engine and transmission may have been swapped out six or eight or more times. As a consequence the engine compartments were designed and built to swap out a power package in less than eight hours.
These days many coaches are available used with a lot less miles. This is being driven by air quality rules that basically prohibit the commercial use of 2-cycle diesel engines and pre-2007 engines in most urban area. The whole state of CA is off limits to the commercial use of 2-cycle diesel engines and pre-2007 engines. It is also being driven by the fact insurance companies do not want to insure for commercial purposes buses much more than 15-years old. Which works out to most buses are leaving commercial service with less than 1.5 million miles.
Since buses do not have to be built to last 30-years and 3 million miles or more the components used are not nearly as heavy duty. Which is why some 20-year old buses are nothing but junk but there are a bunch of 40- and 50+ year old buses still out there doing a great job.
Most newer coaches use a lot of plastic of one kind or another. It saves on weight and the cost of manufacturing. But it doesn't lend itself to longevity. Van Hoole pioneered a lot of those design features which is why so very few older Van Hoole coaches are still on the road--water got in between the plastic and steel framing that has caused a lot of VH's to die of rust cancer.
The European designed coaches also use European designed electrical systems. They may use US designed engines and transmissions that use 12-vdc to run but the rest of the coach uses a 24-vdc system most US technicians find to be an odd way of doing things. Believe me when I say you do not want to have an electrical gremlin in a European designed 24-vdc electrical system. Even if you understand the system it can bankrupt you very quickly and never find the real underlying problem. The MCI, Prevost, and Flxible/Dina 24-vdc systems can be challenging enough.
Another real problem with any of the coaches is getting any sort of parts or service for them. MCI is not too bad with Prevost a close second. But parts availability for any of the others is difficult at best. And even though most use Detroit, Cat, or Cummins engines with Allison or ZF transmissions they rarely can be serviced in truck shops even if they are certified Detroit, Cat, Cummins, Allison, or ZF qualified shops. VH is especially bad about writing additional lines of code into the operating systems that can not be read by standard shop computer interfaces.
Any bus conversion can be a money pit. If you want a really large money pit add a third axle and European design.
At the end of the day a lot of what will drive your choice on which bus to purchase is how you intend to use your bus. There is not a road in this country a school bus doesn't travel to and fro at least twice a day. There are a lot of roads in this country that have height, weight, or length restrictions that will restrict the travel of a coach. But if your goal is to spend a lot of time traveling on interstate highways a school bus, even a school bus set up to do trips at highway speed, may not be your best choice.
Good luck and happy trails!