Greetings and questions about HP and Gearing

Hello all, I am new to Trucking and need the advice of Experienced Drivers. I will be purchasing a MDT, probably along the lines of an International 4400, Chevy 6500, or F650. Maybe a Freightliner. The truck will be a Flatbed with an approximately 20 ft. bed. I will be pulling a Pintle type Flatbed trailer, 24 Ft. with an approximate Gross Weight of 10-14,000 pounds. The truck will have approximately 10,000 pounds on it. The truck will be used mostly on the highway going around 500+ miles at a time. I will have a CDL A license. I will construct or buy a DOT legal Sleeper to put on the truck. My question is, I believe most of this type of truck will be geared for city use. I want a manual trans. What engine and gearing should I look for? The truck will be probably 2000-2005 year model. Thank you.