I cannot see anything to cause alarm. It sounds like a quality of finish issue and not something that would be typical of a home inspection report.
Subject to the laws of physics and land use regulations, any construction project is possible if sufficient resources are devoted to its completion. Whether it makes personal or economic sense to reduce the planar deviation of the floor is something that each of us has to decide for ourselves.
You didn't mention your location, which would be very helpful, as things like frost heave and drainage matter where foundations are concerned.
But, in general, floors are likely to be uneven because of the foundation. Here's where it gets hard, though: the foundation could have gently settled over the last almost-century and be totally fine, or it could be in varying states of disrepair that need fixing. Homes of that vintage may or may not have adequately constructed foundations. (A typical modern foundation includes a wide footer below the frost line; the foundation wall sits on top of this. Old places sometimes omit the footer.)
If it was, for instance, a corner that went bad, fixing isn't terribly hard. You jack up the house, dig down a bit, install solid support, then fix all the cracks and doors/windows you've upset by the jacking.
Alternately, it could be that the entire foundation is bad, which would require jacking the entire house, heavy excavation, new foundation, dropping the house back down, then fixing all the interior work.
Local settling could also be caused by framing rot, though that's usually caught in a home inspection.
I hope you've gotten the impression that there's a broad range of $$$ possibilities here. Get a licensed general contractor that does a lot of foundation work to consult on the job. Not to be biased against young'uns, but if you find somebody that's been in business for many years, they're more likely to have seen similar homes in your neighborhood. (Expect to pay a modest fee for their time. This isn't one of those 'free estimates' kind of tasks.)
If time is of the essence and you have to get an offer in, have your lawyer write massive contingencies into the contract that will allow you to explore the problem and walk away if needed.
Best Answer
Although it is impossible to say for sure without seeing the floors in question, I suspect it is a simple case of poor construction practice. The basement floor is most likely over a concrete slab. The floor may not have been leveled with framing. You did not mention what type of flooring was there, but I suspect the tile or carpet was applied to an uneven base. The upper story floor was most likely built long ago on the existing joists, with the same un-level results. In either case, the only way to level these floors would be to frame a new subfloor level, then install new flooring over a properly installed subfloor. There is no trick or shortcut to fixing situations like this. This can be an involved and expensive project, not a good DIY for amateurs.
As far as the fireplace is concerned, if the room in question was originally designed as a livable space, it is not uncommon to see a small fireplace. Before the days of central heating systems, many older homes (100 yrs ago) had multiple fireplaces. The concern by today's standards is if the assignment of flues is correct. By that I mean, how many and what type of combustible devices share a flue. There are strict rules on what fuels can share flues and an what elevation in a house. This is important as fumes or CO can invade through a flue from one device to another. Again, this would have to be inspected for safety.