Well, not much. I don't have any real experience with this yet, so I'll try to give conservative ballpark numbers. I know the house is brick, 750 sq ft, 2 or 3 bed/1 bath, was built in 1957, and has an estimated market value between 75-85k when fixed up, of course. While some of this info is from Zillow, which isn't known for its accuracy, comparable houses in the neighborhood have sold for similar prices. Without paying for a professional home inspection I have little idea how much all the repairs would cost, but my conservative estimate would be about 20-25k. Unsure how quickly contractors could rehab the place without knowing the full extent of the damages, but taking how small the house is into account it should be doable in 3-4 months (assuming the contractors are reliable, of course.)
This is all still very hypothetical because I have yet to track down the nephew again so I don't even know if he'd be willing to sell for cheap. I just suspect he doesn't want to deal with the property since it needs a lot of work and he left all the trash in the yard weeks ago and hasn't been back since. I guess I should try to track him down first and ask a contractor for a rehab estimate.