You would want to check local zoning to make sure you could build more than one as a lot goes into that beyond just being close to plumbing, electric, and gas.When you mention "a lot goes into that beyond just being close to plumbing, electric, and gas," what are the top things people tend to overlook when preparing to build ADUs?

If I have a specific area I'm looking in I would call multiple agents to ask about the area.I also reach out to communities like this and ask for others' input as well as recommendations for a good crew- handyman, plumbing, etc.Visiting in person sometime in the beginning would be ideal so the neighbors can know you and help give you info when you're gone.And it may just be me but I like to partner with locals

This doesn’t mean anything is “off”; it’s just how I prefer to approach this.To address your other questions, the property listing states that it has a new roof, plumbing, and electrical systems, which I see as a significant plus.

But larger repair items such as appliances and mechanical, plumbing, electrical, roof windows doors etc should still fall under the landlord's responsibility.

Take this opportunity to upgrade your plumbing and electrical if needed. 

Great property, perfect setup for a duplex with separate driveways, entrances, back patios w/ storage, brand new roof, plumbing, newer HVAC, appliances, flooring, etc.

I do know it needs P/B foundation repair, new tile, and plumbing updates.

Or any clues on the document that would allow me to spot any major problem, such as foundation, electrical, plumbing etc.?

If all your plumbing and electrical are in place that would reduce it some but not down to $20,000.

Since the lease was with both then you need to make the check out to both, to do otherwise would be making a legal determination which is what the domestic court is for, and where they can take their issue to resolve.