I added a bathroom (owners suite), took out the drop ceilings in the living room, opened up the floor plan, and new kitchen/ bathrooms/roof/HVAC/Electrical/Plumbing.

New kitchen, bathroom, added an owner's suite to the home, new plumbing and electrical, the HVAC worked but I had to replace the coil and duct, refinished floors.

All of those things can be mitigated by updating electric or plumbing and by supporting the basement with beams or reinforcing sections with newly poured walls.

If there are multiple listings in that area without permits, then there is a higher likelihood of there being no formal permit process.That is so not a reliable method.

Plumbing plus rehab service has quoted $35K end to end to get the job done which is an ENORMOUS cost given my down payment on the home was $55K.

Would I be able buy this property subject-to legally using this method? 

With you and your husbands experience in plumbing I would find a trusted plumber to see what's wrong with the sink and go from there.

Now four years later in the inspection report from my buyer a more dramatic statement is made: "Polyethylene Plumbing Supply Lines have been known to to be randomly defective and can rapture without notice" and the buyer wants me to replace the Polybutylene Pipes in the condo.

Our move in packet has the contact to the tradesmen who worked the plumbing/ heating / electrical on the house.

Where are you all looking for your deals, and/or what methods are you using to get the deals brought to you??