Diesel Bombers - 1989 Ford F-350 Standard Cab Deisel

Hi guys enjoy your Deiselicious writing! My girlfriend is looking at a 1989 F-350 6 wheel Standard cab pickup. Right now I don't know which engine it has. I'm guessing 7.3. It's flawlessly clean, brand new tires all around, has a great 8 foot bed with boxes down both sides. She owns a pest control company and wants it to haul light weight heaters for killing bed bugs. Truck has 83k miles. Couple of questions, 1.) What kind of mileage can she expect around town?, 2.) What's the deal with a 7.3 engine non-turbo? Will this thing move out of its own way, realistically? 3.) I'm gathering from reading about this boat anchor that it will run FOREVER on a gallon of diesel and a quart of oil..??'!! She's needs reliability more than corvette speed or pulling power of a D-9! I think the truck is great BUT he's asking $7,000.00 and THAT sounds sky high. What do you'all think? We're not in a big city but near one.