I was in a similar situation about a month ago.
I ended up purchasing an 06 dodge ram 1500 big horn edition. its 4WD I get 18ish mpg on the highway so just shy of what you want in 20mpg. It is also a quad cab.
I purchased it for 14,300 which is about 4-5 grand under what its worth. So yes I did get a steal on it especially since it only has 60,000 miles on it.
In my opinion, just narrow down what you want. Personally, I am not a ford guy and I will never buy a ford truck... EVER. I personally dont think they are reliable. I knew I wanted an 03-07ish dodge, chevy or gmc 1500. Just go out, "kick tires" as they say and drive a few and figure out what you like.
I also ended up with this truck because my stepdad has the same truck just a year newer and has always had a dodge truck and loved it.
Thats my two cents, take it for what its worth.