Difficult to answer all the questions...
GC or no GC: how much time do you have? How much experience do you have? Do you have a decent network of subs or labor that you can use? How quick do you want it done? How much of a savings is worth it to you and for your time?
Nothing wrong with going all gas.
Typically metal roofs arenít replaced just to replace them. The aesthetics go well before the structural integrity starts to fail.
I wouldnít raise the ceilings. You will be surprised what other work has to be done to gain a foot of ceiling space. If you just do it in say the kitchen and living room, not a bad idea and you donít need to replace the roof.
Pex is pretty much used in all the homes and remodels that I am aware of.
I would say that some of the biggest advancements is in the technological options available in houses now. Plumbing and electrical are pretty much the same as they have always been...just a lot of accessories that we used to not have.
HVAC would be a big sticking point for me on a house with that layout.
Good luck and feel free to ask if you run into any issues