I'm going to have to do some framing, drywall, and basic stuff, but I just don't want to run into some junk about how something isn't up to code and blah blah blah.  

-----------------------------------------------------------------First things first: What does the building code say about trenchless re-piping, about the materials that can be used for a sewer line, or about relining?

Spend $1000 and do a postcard campaign to that zip code. 

The city is now requires either of the two below prior to allowing for renewal of your rental license. 1) A lead safe certification (Valid for 4 years)2) Or A lead free certification (Permanently valid) YOu will not be able to renew your rental license in many Philadelphia zip codes without one of these certifications which has to be uploaded  this certification to the Philadelphia Lead Certification Submission System (link below).https://leadcertification.phil...Site below gives you more information along  with deadlines by zip codeshttps://www.phila.gov/2019-10-...Hope this is helpful.

Here's my (similar) story: Habitual late payer, usually finds a repair needed, withholds payment, threatens to call codes, has added another tenant to the unit without notification, a HUGE pain in the butt to the neighbors...

They had no idea how to classify our operation since there is no short term rental regulations in their housing code.

If there are any unrecorded liens (ie code infractions, unpaid utilities, etc) you'll be responsible for these

On the note of topics and code, depending on when the house was built if you replace the dry wall you lose grandfather status and will be required to update both the plumbing and electrical to modern standards.

I have a duplex offer accepted near Waldo, great location zip code 64113.

If you crack the code I’m definitely all ears and hopefully you do.