Contrast that with a normal rehab where you can often get certain aspects of the home grandfathered in and usually only need one or two specific tasks done by licensed professionals. 

@Alvin SylvainSilver has natural antibacterial properties.

These have a finite shelf life and tend to degrade form the inside, so you know you have a problem when you see water, a high bill, or bacterial growth.There's not really a one-size-fits-all answer here, but rather you can learn through the process of doing and figure out which ages of homes you are most comfortable with OR you can invest in new construction and newer homes. 

Usually, this has been in states that were relatively anti-landlord.

@Sam Bagwell  well can point you in the write direction just don't tell anyone on BP its pretty much anti paid training around these parts...

Up or down market it doesnt matter you have to buy low and sell high.I buy 3-4 properties a month and is a very daunting task.

Write down all the tasks that are part of your work flow for property. 

Assume that contractors will ALWAYS choose the cheapest (and ugliest) material for a task.

I recommend Jim Suebert of Pride Cleaning and Restoration for that task.

Open to other ways of accomplishing the task and helpful hints on where I should start analyzing.