The surface beneath the current floors is a concrete slab that looks like used glue to hold the parquet tiles down.

Owners trying to rehab themselves often don't know little tricks such as adding a spoonful of dish soap to their mud to help eliminate air bubbles (and thereby, an additional coat), or that the process of coalescence in exterior paint is negatively affected by an ambient and/ or surface temperature below 50 degrees.Being from Cleveland, unfortunately I can't much help with the Milwaukee area.

Overall the place is in pretty poor conditions to my eye and with all of these issues I've seen on the surface she has the property listed at $125,000.

I was a lurker here on these forums for a bit but have now surfaced.

If house was sealed up it isn't uncommon and may all be surface level, you may be able to treat and seal.

The boards are soggy on the surface.

Your local building inspector (city/county) will be able to advise.A little surface rust isn't bad, but if the conduit has rusted through, you'll need to replace it. 

If you couple foundation issues with a C- property that needs to be brought up to a B neighborhood I would not think $25,000 would scratch the surface.

As others mentioned, the fact that it's been on the market for nearly a year, at what appears on the surface to be a decent buy, seems to point in the direction of deferred maintenace and a big CapEx bill coming down the line.  

If that's the case, a final inspection is a surface inspection completed by a licensed inspector in that jurisdiction.