For bluebird, I use this.
Unit Dashboard | Portal
The specs it gives are things like brake type, engine serial number, transmission serial number, rear axle ratio, headroom height, length, etc.
The serial numbers you can use at the manufacturer's respective site to find out engine model, hp, trans model, etc.
All important if you're wanting to get the perfect bus. I found a bookmobile that I was really interested in, until I found out it had a isb cummins with an at545 and juice brakes. All 3 things that wouldn't particularly be deal breakers on a $2000 school bus, but on a $10000 bookmobile, I expected better/more for my money.