We seemed to have lost Drrtybyl, but it would be nice to know the price range.
Let's start with the towing requirement, and the desire to have a small pickup for mileage. An F150, C/K10 or D100 is EXACTLY the same size as an OLDER F350 (chagned with the StupidDuty), C/K30 or D300/0. You can have a reasonable size truck AND good mileage by sticking with the '90s versions of these trucks in one ton, single wheel format. Once you get to the one ton size, it can easily be fitted out as a decent tow vehicle. I set up an '03 Dodge 3500 for my best bud, single wheel, put on 19.5 forged alloys (Rickson) and run 225-75 10 ply rubber. You can stiffen the back end up with air bags, and run really good shocks (Billstein). Amongst several other things, it pulls a four horse trailer quite well.
If towing is the real deal, though, it is impossible to beat a REAL TRUCK rather than idiotic little toys. IHC, Freightliner, etc. make perfect mid-size, "lo-pro" vocational vehicles (in 2WD only, or I would have one instead of an F450). These things have real axles, brakes, transmissions and most of all ENGINES. Instead of crowing about the miracle of 200k, you think more in terms of million mile life. Believe me, I tow heavy with one of the "big 3" POS, and it costs a LOT more than my class 8 semi to keep on the road (and not much different in mileage). There are also Japanese alternatives (cabovers from GM/Isuzu, Mistu, Hino, etc.) but they tend to be a bit light on the horsepower end. If you are into really exotic stuff, I have a D220 Iveco (sold in the US in the late 80s or so) that I have pulled big loads with. At 120 HP, not much happens in a hurry, but the fuel economy and reliability are awesome. There are still a few around, and they are still supported by the factory (Fiat).
Finally, there is the business of engine swaps. While it might seem neat having a GM two cycle under the hood, if you ever put fuel in one, or listened to the racket running down the road, you would realize that they are not in any way useable these days. They are also EXTREMELY heavy. We used to bring in Japanese diesels in the days before there was much else around, and my absolute favourite conversion was 6D14T in F350 (starts at the rad, ends at the dash, requires raising the cab off of the frame - but HOLY SHEEP SHYTE BATMAN, DO THEY EVER PULL!!). 6BD1 Isuzu is another one that really belongs under a daily driver hood (it is a lot smaller and lighter than the big Mits, but still very powerful and genuinely commercial in reliability). But, these days, I would have to question why anyone would swap in any engine except the Cummins 6 bangers. They are so cheap, easy to find, and fantastically supported by the aftermarket, there just isn't anything better around.