I view buses like big rigs
Not quite, buses have medium duty engines and, while they are built similar I wouldn't expect to get the million miles that is common on the much larger semi engines. If running good I wouldn't be afraid of a 300,000 mile bus, but I don't think I would buy one with much over that.
I had two semis, one with a Series 60 Detroit and one with a N14 Cummins. The Detroit had 950,000 on it when I sold the truck, engine still strong. The Cummins had 1,500,000 on it when I retired. I did put rod and main bearings in it at about 900,000 but, wouldn't have had to, I just put the in because I had it opened up.