I'd focus on getting a better property manager, their job is to shield you from most of these headaches and to prevent them in the first place through tenant selection and doing regular property inspections.

Moving forward, ensure that all necessary inspections and reports are obtained and reviewed early to avoid last-minute surprises.

To build that kind of portfolio you will also need help with financing, locating, inspecting, repairing, etc.

I am a licensed Lead inspector in New Jersey for anyone who needs an inspection to get compliant with the new lead law in NJ. 

Side note, The calendar can get tricky to work out inspections, appraisers and closing dates as the home may have guests.

Even though I listed it as-is... we had so many inspections it was truly mindboggling. 

You are not obligated to make upgrades -- especially if they saw the property and walked it before they signed the lease and or if they sent you back the property inspection sheet from when they first moved in and did not request anything be repaired or upgraded.

They want to do everything by the book, but don't understand the inspection process and don't want to invite an inspector to come over and throw the book at them.  

Inspections can also cause delays, especially if unexpected issues arise that need to be corrected before moving forward.

you can also run into an appraisal issue as the appraiser will request loan program and may keep the inspection "Subject to" until a permit or approval is given from County.