If 'duplexes' are selling for $X I'd want probably 10-15% off of X to cover the risk I'm taking and eventually the cost of converting it to SFR if I had issues with code etc.

I'd stay out of 53206 zip code.

Yes, people still operate them but one noisy neighbor and code enforcement can turn your investment into a nightmare.

Additionally, there's limitations in building code that do not allow builders to build on lots that once permitted muti unit zones, that's why you'll find a lot of older non conforming multi-families in the Greater Boston region. 

In your situation, the investment is strictly for the purchase of rentals (buy and hold), therefore, you'll have to connect with a commercial/portfolio lender to finance the deal.

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?

Simple and easy to do.Not sure what kind of workarounds we can do since our loan was securitized into CMBS and they seem to be pretty strict and not able to work "outside of the box."

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

The lease signed by one of my inherited tenants contains a strict non-smoking policy that includes a clause citing a $200 fee per incident.