Couple obvious questions - What's wrong with the floor plan already in
place, i.e., the original design ? Can you figure out what that was and
go with that ?

Second, how much money are you going to sink into this and is it worth
it ? If you buy a house for a 100k, put 100k into it, and in the end
have a house that's worth 150k, you're upside-down. I should know !

Ditto the other comments, if the house is really weird, like it was cut
up into apartments, you want to determine in general where things
should be and which walls are add-on, which are original. Which walls
are load-bearing, which ones are not. Where the plumbing runs,
especially the stacks. Precision does not matter during that part of
the process, if you hire a guy to come in and put walls where you want
them, you don't need exact measurements.

Your bigger issue is the foundation and the roof, the floor plan is
secondary at best to getting that big stuff squared away.