I have an idea..lets call it old home character.
Old homes are prone to these issues says obvious-man.
My suggestion would be to post some pictures or get a professional to look over what could be multiple issues. You could have pier settlement, termites, dry rot, leaks, or just plain undersized joists for what you are supporting.
I would have someone look at it, then tell us what they say....we can then help out with solutions. And also tell if you are being sold an fresh, clean, iceberg from Florida.
Tell us how you make out.