176k mi is NOTHING make sure he changed the oil every 3-5K mi and that he DIDNT baby it. these trucks dont work well when there babied, carbon bilds up and nasty stuff happenes.
If you get the truck, its gonna take a lot of getting used to and require a different mindset.
5 speed, your lucky. some would give a lot for that. good solid tranny.
mpg potentials with the 7.3, not a lot. max, maybe 17 or 19. MAX
with CC LB m getting around 13-14 but i drive her hard every day.
as far as exhaust goes, if you have a welder and a grinder, get a downpipe off the net and just straight pipe it yourself. youll save $300
intake, go with the DIY, I got it, i love it.
chip, go with tony wildman. if your only looking for econo, get only 3 positions, stock, econo, street (or tow, depending on what you do)
you wont be disappointed when you get er. I love mine to death, but she is needy, you cant just use and abuse her like a 90s honda.
Hope this helps, if you have any questions, feel free to pm me or post up more!