It doesn't have to be used suspension. I put airbags in my f-250 and they make a BIG difference. I rigged up my A/C compressor to be an air compressor and run a regulator on the bags. SWEET!
I wouldn't even consider the 4 banger. My cousin gets pretty good mileage with the v6. I will call her and find out exactly what kind of equipment she has in her truck.
You could remove all but one leaf spring and install coils. That would lower it and give you a pretty good ride. The front springs can be cut/modified. You can do whatever you want to a truck suspension. I have some new and used springs you can look through. You could use sway bars if you wanted to. That would do a lot.
My cousin has a 2002 4x2. I think it is a 3 liter, but will have to check. Not sure on the rear end either, but it goes pretty good. They are real good for aero modding.