Sounds like you could be in over your head here, man...
Well, on a similar note, I think I only said "Yes" to its disturbingness because of the things you found in there (since my dad has been in the business of snapping up old buildings to manage/sell, I've seen my fair share of dusty old haunts).
The worst one though, without question, was this closed-down Chinese restaurant on the same block as the rest of my dad's buildings. I had passed it by all the time and always wondered what it was like in there, but I think I was better off not knowing. I think what happened is the family (the Lees, I believe) lived in the back of the restaurant (there are multiple rooms--we turned it into a mini-student housing bulding now), but they were forced to shut down because they had horrible sanitary conditions and were allegedly raising livestock in the back rooms (there were multiple "Best Take-Out" awards lying around the place, so they must have done something right at some point). When they were told to leave, they left. As far as I can tell, they didn't take anything.
Some alarming things:
-Multiple hidden packets of what may or may not have been some kind of crack/meth (the Lees were/are apparently druggies. We think they also took a dump on the back porch of the place after we negotiated the deal because, well, that's what was there).
-The kitchen, literally covered in a thick layer of grease.
-Several dead rats--one in a wok on the frying pan.
-Rat nests under things.
-A door with a latch on the outside and scratches on the inside (this totally freaked us out until we deduced that was probably where they kept the animals.
-Wiring in a light that was apparently just two wires hand-fused together (the ceiling above it was lightly scorched).
-Needles above an outside freezer of sorts.
-Copious external grafetti.
You wouldn't know it now, though... the place is really fixed up (my family/the local college who owns it now did a ton).