It's gas, there isn't a diesel option for the '04-'08 F150.
I have an '04 F150 Lariat Supercab, I bought it new and 80K miles later, I'm pretty happy with it. Most of my commute is highway miles, and I average 16-17 mpg. 19-20 mpg is the best I've ever seen out of it, that's unloaded on a long highway trip.
At that mileage, you'll be coming up on a tune up and depending on the build date, you can be in store for some serious hell. 2004 to early '07 had a goofy two piece spark plug that would get stuck in the cylinder head and break off upon removal. There are tools to remove the broken plugs, but in all honesty, if you have it done at a dealer it's gonna be expensive. And in an absolute worst case scenario, the cylinder heads may have to come off to drive the broken remains of the plug out.
I haven't had any major, leave-me-on-the-side-of-the-road problems with my truck. A few little annoying things, squeaks and rattles and such, it's kinda hard on brakes/rotors (the rotors seem to warp rather easily, causing a pulsation). The 18" BFGs on most Lariats are kinda so-so, I got about 40K miles before they hit the wear bars on my first set, this second set seems to be better. I've had to replace a window regulator, they're cable driven and the cable can break, it's a pretty common thing on those trucks.
The spark plug thing is probably the biggest hassle, but in mid '07 they changed the cylinder heads and went back to a conventional style spark plug. If you can, find out the engine build date, it'll be on a tag on the driver's side valve cover, I can tell you what plugs it should have in it by that.