For the highway speeds to be close to 65-70MPH you will want to find a bus with highway gears. Most don't have them since they mostly do city driving in stop and go. You can change out the rear end though to get a higher top speed. The AT545 is the bottom of the line Allison, it is very common but I would not buy a bus with it. The MT643 is a much better tranny with an overdrive. If you can find one with a retarder you are even better off. From what I have heard the Cummins 5.9 is an ok engine in a bus but they don't hold up very well. They are like the AT545 cheap and widly used. I don't have the specs on hand but a Cat 3208 or my favorite Cummins 8.3 would be best in a bus. For more info you should email Greg Archambault, he can tell you almost anything about school buses you could ever want to know. You can get his email addy from his website.
http://flbus.tripod.com If you are looking to buy he is a good honest guy who can find almost any buy you want. He is on the road a lot so you may have to wait weeks to get a reply email from him though. HTH