Uneven Floors, Red flag?
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.