I'm sorry to say I think this is a poor idea and, at least in my area, it would screw your resale chances right to the wall. What with there being no access to the downstairs bath from within the main living space, you would essentially be turning your main house into a 2/1, which is widely considered obsolete and less desirable. Since you have to go outside to get to the other bath it might as well not really count unless you are trying to set it up as an apartment with a separate entrance. If you can wring out a 3x4' nook for a separate powder room somewhere (so that there are two toilets on the main level) that additional half bath would gain you something back - although nowhere near as much as you lost in disposing of a full bath.

Spiral staircases have serious problems, the least of which is getting things up and down them (forget furniture, just stuff like laundry), and people are more prone to falling because of the wedge-shaped steps. You need to have a big enough staircase so that people can step easily on the wide ends of the treads. As a homebuyer spiral staircase would be a deal-breaker for me, since I don't want to get into the renovations involved in removing it and reconfiguring a new staircase.

I really think your first expenditures should be a quick spiff-up of the master bath to make it usable and then having a sit-down with an architect figuring out how to reasonably provide access from the main part of the house to the downstairs. You have a gold mine of wasted space there! One notion that crossed my mind is to consider putting two or three nicely-sized bedrooms (using egress window wells to satisfy the building codes and provide lots of natural light if it's not a walkout basement) down there with that mysterious random bathroom. Maybe even a nice big family room, too. Boom, you have now doubled your living space and given your marketability a great big kick in the pants. Then, and here's the yummy part - reconfigure your two bedrooms and two baths upstairs into a generous (but not ridiculous for the house's footprint) master suite with a master bath, nice big closet, and a powder room for the use of visitors. Part of that space could have been scavenged off into a stairwell. So now you have a 2000sf, 3-4 bedroom, 2 1/2 bath house with a real master suite instead of a 1000sf 2br/2ba. Now you have much wider appeal for resale, and you haven't affected the aesthetics of the neighborhood the way "pop-topping" the house would. There are mortgages available that are based on the value of the house when the renovations are finished which you could potentially refinance into to provide the funding for the project.

My DH and I debated this very issue for several months with the only difference being that we have access to our downstairs from inside. In the end we decided that two small baths (5' x 8') were better that one large, and for the 2-3 months it took to finish each bathroom I realized that I would have gone insane with just one bath on the main level (even though one HUGE bath sounds so wonderful to me). Its just the law of nature that someone is going to need the bathroom the instant you settle in the tub for a long soak. We have 2 small children though, and when they are sick or they need to potty they need to go RIGHT NOW, not in 5 minutes after they run downstairs. So after our renovations and as a mama I would never even consider a house with only one bathroom on the main level, maybe a full bath and a powder room, but definately nothing less.