1. I don't think the MS2000 will be adequate from 40 miles away. Being at the top of the hill is a plus, but I'm still not sure this antenna would be up to the task from 40 miles.
2. Almost everything on the TVFool report for your zip code is within a 25º arc. I think directional would be better here, and you may not even need a rotor if you aren't going to try to pull in out-of-market channels. Most directional antennas can handle a 25º arc, as you can aim an antenna to be no more than 12-13º skewed from the tower. Aim a directional antenna at about 0º, due north (about 9-10º magnetic) and you should have a good shot at everything within that arc. I think a 4228 might be a good choice; the 91XG might be a little too strongly directional. If you don't care to try for the most distant channels from other markets, I don't even think a rotor is indicated here.
3. As Don M. mentioned, multipath is a very real concern with omnidirectional antennas, particularly in areas that are hilly and/or have a lot of tall trees and buildings nearby.
4. Unless being on top of the hill provides a lot of additional gain, a preamp may be helpful here -- especially if you'll be splitting the signal into the house. But by all means, try it without one first, and if it's good enough without it, save the money. If the signals are weak, buy one. I don't think you'd have any overloading concerns. My TVFool results are mostly in the -102 to -108 range, and I only needed a preamp with my 91XG because I wanted to split the signal three ways.
5. All of your Pittsburgh digital locals are UHF. After 2/17/09 there will be some digital VHF stations reachable (channel 5 and 7) but these are both CBS and you'd already have that already covered with KDKA so you may not want to bother with VHF, especially since that would require more antenna hardware (or a combo antenna for 2-51) and a rotator.