I built up a 95 Geo Tracker for really inexpensive. Not a lot of power, but it went everywhere I wanted to go. Top speed on the highway with mud tires was about 70. I was still getting around 25 MPG with a 4.5" lift and oversize tires. You can get a four door Tracker or Sidekick, there are a lot of aftermarket parts for them now. If you want more power, drop in a 4.3L Chevy, or a Volkswagen Diesel.
I love my 74 Scout II, but it only has a 3 speed in it, so top speed is about 65mph with stock running gear and tires. With the 258 6cyl, I am getting around 18mpg, but I will say it is very torquey.