You have competing requirements, which is not unusual.
For highway driving you want a big engine and decent transmission. They tend to be the province of rear-engined buses, ruling out a rear garage. It's possible to find a conventional with the Cummins 8.3L, but they are rare.
You want 40', but few of them are SPED buses so no lift. Buses with lifts tend not to have the better power-trains as they rarely venture on the highway.
So you have to decide what's most important, and where you can compromise.
A DT466 with a decent transmission would be fine for most purposes, and there are a lot of them around. Avoid the ones with the AT545 transmission. That trans ain't great, and they only go behind the lower-powered DT466 anyway.