my 4wd 8200 lb 6.0 powerstroke excursion regularly gets 18 mpg tankfuls on the highway. I can do 14 mpg towing a 10k trailer. mixed city is usually 16 or so, but I have done over 22 on the highway if speeds are kept low and I stay behind (not necessarily on the ass of) a large truck.
The 2wd's I guess get a bit better mpg, less weight, lower to the ground.
I know several people with 7.3's that have also had highway tanks in the 20's.
I wouldn't ever want a manual transmission behind my diesel, and few others do too... thats why its so hard to find one (and I think dodge is the only one still even offering one).
I know lots of guys with diesel trucks. I cannot think of a single one of them with a manual transmission that hasn't been through at least one rebuild before 100k. thats pretty much reality. The shifters on these trucks have a throw measured in yards not inches, and going through the gears unloaded with 500-600 ft lbs of torque is not very rewarding. Most of the time you can start in 2nd and skip shift to 4th.
As for the autos, the allisons are pretty famous for needing some work to be reliable. I've seen quite a few with over 100k that were cheap because they needed a new trans. If you want to turn up the boost, you will probably need to spend some cash upgrading them. Diesel power has covered the allison pretty extensively.
the ford 4r100 is a total piece of E36 M3, I know people that have done stock rebuilds every 40-50k. If you tow or plow they just die. If you leave them alone you are lucky (extremely lucky) to get 100k out of one. My truck has the 5r, its a LOT stouter than the 4r. For a stock or slightly tuned truck it will be reliable just the way it is. If you want a 500 hp truck it will need work, but then all the autos will need to be built for that. If you want a 500 hp truck and you want a manual, you will probably be looking a clutch that's not very streetable and replacing transmissions every so often.
For the engines, the new emissions era trucks all are extremely complicated and come tuned much closer to the edge than they did 10 or 15 years ago. You can't add a chip and get 50 hp like you could before (not without blowing things up anyway). EGR coolers are unreliable. Diesel exhaust fluid is a pain in the ass. Regenerative cycles kill fuel mileage.
Personally, for the premium the diesel trucks command, for your stated goals, you are gonna be a lot happier with a gas truck. The gas truck is lighter so if you lower it and hypermile you can get fuel mileage rivaling or exceeding the diesel, and you don't need all that torque if you aren't gonna be using it for heavy hauling and towing. Gas is much cheaper than diesel, and the maintenance on a gas engine is a quarter to a third of the cost of maintenace on a diesel, and the new diesels are fussy about regular maintenance. Gas engines warm up faster. diesels are slow to warm up and all the newer ones dump fuel until they are warm so they get really bad MPG until warm. on a 40 degree day it can take my 6.0 5-10 miles to hit the thermostat.
I think the best engines to work on, drive, beat on, etc, are the late 90's offerings. I'm not a huge duramax fan, just have not seen much evidence that they are more reliable than the powerstroke or the cummins. The cummins are all durable but your best bet is a 12v. dead simple and easy to work on and will make as much power or more than any other engine out there. all problems are known and all solutions are a ups box away. adapters to any transmission known to man are available as well should you chose to drop this motor in your wrapper of choice.
The 7.3 ford powerstroke is a great, reliable motor with good power potential too. the injector fire system uses a high pressure oil pump and is less reliable than the older cummins. Sadly they come sadled with 4r's.
having owned a few diesel trucks and having had a half dozen or so in the family, as well as knowing a ton of peple that run them, I think if you have never owned a diesel before your best bet is to try and find an older used one to start with. The old ford 7.3 idi non turbos get pretty amazing mpg. I had a 91 ext cab long bed 2wd with really low gears, maybe 2.73's, and it would run 70 mph at 1200 rpm and get 20 mpg all day. not bad for a giant brick. I've seen great deals on these trucks too, $1000 for a running 2wd with higher miles around here. thats about half of what your "intake, chip, and exhaust" will cost you for a new cummins, pstroke, or dmax.
My brother has a twin turbo common rail cummins with the NV 6 speed, triple disc clutch, and a ton of modifications (all new fuel system, dual pumps, head studs, blah blah). In 4 hi it will burn the tires at any rpm, any speed. You can hit it at 70 mph on a straight stretch and spin the inside front tire. 2wd is an exercise in self restraint.
another guy around here has a black triple turbo newer chevy, looks like a total street truck. He likes to take it to the truck pulls and run exhibition after the built diesel trucks pull (the ones that come in on a trailer) and usually outpulls them. lol.
But no matter what you do to them, they aren't that fun to drive. Not like a car is. The steering systems are an older design with a ton of play. No matter what springs and bushings you use, you still have a heavy body on a heavy frame.