Saint Louis has it's problem areas. I saw a building that had squatters moving furniture into a condemned building. I have had 3 AC units stolen on listings before close. Its a real problem, and COVID made it worse, since our Police are already spread thin and quitting left and right.
You can mitigate some of the risk by:
Using PEX tubing for the plumbing for anything you replace. Really good for whole system overhauls.
Put a cage on the AC condenser or put it on the roof (if flat).
Put the lockbox in a non-obvious area.
Bar any side doors from entry until occupied. Deadbolt and a kickstop work wonders.
Keep the lights on in the building. Keep it clean, picked up, and looking lived in. It helps a lot.
Talk to the neighbors. They want the problem properties fixed, and can help keep an eye on the project.
Get an alarm. BP yells about Simplisafe a lot, or something similar. Low cost but loud if someone breaks in.
Ultimately, it can be a lost cause in the super shady parts of town. Once you start researching STL, you start to see these pockets where there are unimproved buildings next to finished projects. That's where the money is. When the finished projects set the ARV high, but the unimproved building hasn't caught up yet.
Cheers.