You haven't really gave us much to go on mate, for example, you going to be using to truck as a daily driver, gonna use it for light towing, heavy towing, weekend four wheeling ect?
For starters I would simply suggest any F-150 with a non-triton motor, for fuel economy, should be pretty easy to find a newer one around $20K, although a good all around truck would be a F-250 with a powerstroke which would pretty much leave all doors opened for ya, and they are common enough that a decent price shouldn't be too hard at all to find. I can say this also, my father owned a 04' F-150 with the 5.4 triton, and while the gas wasn't great (it is a truck afterall :P) it was driven all over the US while the had it, before they traded it in last year, it was over 400K miles on it and as I recall it had no problems. I tried to give him $9K for it but he refused and then he got less than that from the dealer

, althought any diesel motor can easily last 500K miles if you take care of it and I don't mean just changing the fluids.