Wow, those were very thorough answers! THANK YOU!
Some follow ups:
The existing houses are definitely not in a condition to repair, but the foundations (from what I could see walking around the outside) didn't look to be in terrible condition. They are old (1950's) but the concrete doesn't seem to be cracked like a lot of the contemporary foundations seem to be. Are there any benefits to rebuilding from the foundation up, or is that the house is coming down, remove and rebuild the foundation and start from a green sheet.
If there is asbestos mitigation that will have to be done, and the property is worth less than 0, how do you convince someone that getting rid of it is better than letting it sit and rot?
One of the properties is actually sitting next to an open lot. If I were able to get both the lot and the house, how easy is it to join plats when they are both drawn out with build lines on the drawings that are filed with the city? I'm assuming this is something that can be done, but what's the process? Just call the city and say you want to join two plats to build something bigger? They are zoned to allow attached single family housing, so in theory you could redraw the lines, but how common is this?