Drop of water depends on where the water is. Watch online videos to diagnose problems, it's the opposite of doing this for personal health. People have some crazy cheap ways to address issues.

Crack, draw a line parallel to the crack on each side. Measure it and it on the wall. Check back and see if the measurement is changing. Patch it if it's stable. If it's not stable, probably not good news, start with videos of what others have done.

Ants are the easiest. There are barrier sprays that will keep out any insects. Fall instructions on the product but typically just spray it on the outside of the foundation once a year.

Houses are over engineers to stay standing up so don't worry too much. The expensive stuff will come around ever couple decades like roof replacement so plan for that but most everything can be remediated with little effort, especially if you don't care how it looks. You said it's a fixer up, each patch is just another layer to its character.