IH sixes are very good engines, right up there with the Ford 223 or 300; and the Chevy 235. Very durable, my 67 Travelall has approximately over 300k on it's engine and no known overhauls; I'm the 2nd owner.
Like anything mechanical, dependability can depend on previous owner's maintenance; or lack of it.
Mileage you'll see is probably gonna be around 12 to 14 mpg depending how deep your foot is in the throttle, axle ratio, and the weight it's pushing.