Sounds pretty close to what I am in the middle of building right now. I have an 03 GMC 6500 with an 8.1 gas motor and Allison. Got it as a cab chassis. So far I have the truck all refurbished, painted, and fifth wheel installed and ready to tow. Just picked up the materials to build a 7' box behind the cab as a sleeper/backseat (all I have room for without stretching the frame).
We took it on the first trip last week (no box) with my 40' enclosed trailer, 500 mile test run. The trailer is a gooseneck with fifth wheel conversion, 23000# loaded, weighed the total unit at 33500#. It was a little under powered, but I am probably just spoiled, I could only keep up with the semis on big grades, I am used to passing them. Other than that it handled the trailer great. Very stable, heavy chassis, big brakes and 22.5 tires put my dually to shame for handling that big of a trailer. I still need to add a rear bumper and receiver hitch so I can tow a regular trailer, but I am sure I won't even know a trailer that small is behind me. I really couldn't feel the big trailer except for weight on the grades.
No idea what kind of mileage you can expect from your diesel, I am sure someone else on the forum can answer that. Mine has a gas motor and a 5.57 gear. Runs at 3300 in 4th and 2400 in fifth at 65 mph which is just right for the motor, and only 100 rpm or so more than my dually with the same motor. Just make sure your truck is geared to run the way you want it too, some of those trucks are geared for local deliveries and are really limited at highway speeds, and others are geared for the highway and trade off power if you need it for your driving conditions (in town, hills, etc.)
As to $$, the price seems high for the age, but great for the mileage. It will come down to the condition of the truck, and what you are getting in the "living quarters", but does not sound bad at all.
I paid $4800 for my cab chassis, a 2003 with 180,000 miles. By the time I did paint on the body, blast/paint chassis, new aluminum wheels, new steer tires, trailersaver air hitch ($1500 used), lighting, air ride seats, material for box, and minor maintenance, I've got $14000 and a LOT of work in it. And yes Blizz, I did count all the trips to the hardware store and Home Depot. That does NOT include any sort of living quarters type items like plumbing, heating, etc. as this is just an overgrown sleeper now, and not the full living quarters I had wanted to do. So you really can't build that one any cheaper than you can buy it