1.) It's a close call, and you could do either first. If you sand first, I doubt you will hurt finished floor too much when it comes time to paint (as long as the floor is sealed, dried latex paint will come off easily with a fingernail--just don't spill a paint can, though). On the other hand, if you paint first, you won't hurt the walls with the floor sander because to get near the walls you will be renting something called an "edger sander." It's an orbital sander similar to those you buy off the rack at Home Depot but much more powerful. So, you'll have quite a bit of control with the edger sander.
However, given a choice between walls first or floor first, I would choose floor. The reason is, from an aesthetic perspective, floor finishing is permanent, walls aren't. What I mean is, you want to get the floor sanded down, stained (optional), and sealed to your liking. The floor will have a certain look--gloss or semi-gloss, a certain hue--that will help you determine the color of the walls. It's much more difficult to do it the other way around.
2.) Fireplace and chimney would probably be toward the lower-to-middle end of the spectrum. You have two jobs. One is masonry and the other is carpentry to accommodate the chimney within the house. Even if you're putting the chimney outside (the smarter choice), you've still got to punch a hole in the wall for the fireplace portion. But what really drives it up, in my opinion, is that masonry is not one of those highly competitive trades where you can negotiate a great deal. You may have to take whatever estimate you can get. Still, I can't imagine it would be up near $50,000 at all. If you can stomach it, one of those metal-ducted chimney boxes like you see on newer houses would be much cheaper. But I can certainly understand not wanting one of those.
