Is it "Ryan's Office" or will both of you be working out there? I ask, because you say you want a full bathroom: For a guy's shop/office, you only need a urinal. They take up a lot less space, you don't have to add a whole room, just a partition. You make a lot more trips to take a leak than the other.
For those times when you need to "sit down for a spell," make a trip into the house...turn it into a snack run.
Unless MC will be working out there too. Then you'll need the full deal.
Urinals are quicker and easier to clean, too--toilets always seem to be really disgusting.
You might not be thrilled about the outside stairs, but as you said, it will free up a lot of floor space--both inside the shop, and upstairs too. It might be possible to do an old-style covered stairway outside.
If you do the stairs inside, you can put a generous storage closet/air compressor closet under the stairs, and have a storage nook under the lower portion of the stairs for engines, etc. You could also stash the HVAC handler under the stairs.
I have a 32x40 shop, 12-ft walls with no support poles in the middle. For $80 per truss, I could have had the rafters opened up and had a "bonus area" for storage. I have 22 trusses, so that would have been expensive. Plus, I've seen guys' shops with upstairs storage, and they always end up stuffing it with junk. I opted not to spend the money.
However, I wish I would have done at least 11 of the trusses, because then I could put my office up there. Instead, I've lost a 10x10 footprint on my shop floor, walling it off and building my office there.
It will be cheaper for you to have someone build the outside, rough-finish the upstairs and then you guys can finish it off yourselves, and finish the shop-portion yourselves as you can afford it. If you look at it that way, you'll get a bigger shop, it just won't be ready to move in the day the construction workers leave.
-Brad