That truck has 300,000 miles on it, and wont get 20MPG unless you put about 2000 into it. If you do away with your budget, or knock your 20 MPG down to 15 MPG its doable. getting 20MPG is hard for a 4x4.
It's a 2003 4 door short bed has 200 thousand which seems like a lot but a well maintained 7.3 can go 500 or better. It also came out of Texas has no rust some dents nothing major though still has factory exhaust original and air box has had no high performance mods completely stock. Drove to fort Wayne over Halloween week end and got 19 mpg oh it does need a fuel sending unit fuel gauge acts goofy sometimes.