Hay Truck Info

I have a Hay farm in Oregon and untill now have had all the hay hauled buy the pros. I now have a trucker on staff and want to get a truck to haul our own hay. I need help deciding what I need for a truck.(ie axle ratio,Hp,preferance on brand. but mainly the gear ratio) I will set up the situation. We will haul about 100 loads per year, I have a set of trailers, they are a set of doubles 40' and 24'. So I will get the loads up to 105,500 lbs total. the hay will be hauled mainly to one spot. 450 miles away. we dont plan on doing any back hauls. If it makes any differance we will be starting out at 5000 ft elevation and ending up at sea level, so it is mostly downhill, untill we go back but plan to be empty. There is one pass we will be going up about 1500 ft in elevation, I think the sign says 6% grade. I have asked our current drivers and you can quess each one had a differnt answer, I think that is because they haul all sort of loads in all sort of locations. So my questions are here

What gear ratio would work best in this situation? or does it matter?
Is there a certain Horsepower I need?
I am going to buy used, what price range would a reliable truck that fits my needs be in? I will need a sleeper cab.

Thank you