Since there are some obvious repairs needed, once the inspection is complete, what sort of repairs or cost would drive this into something we should walk away from?

I assume you will have no credit and no work history graduating high-school so if you were looking to buy in the near future, you would need a partner of some sort.

Some sort of private lender, perhaps?  

My house hunt so far has been focused on using another FHA loan since my family has expanded to get another duplex with at least one of the units having three bedrooms, and I would be able to keep my current FHA financed duplex that will cash flow easily once fully rented after we move out.Across the street from my house is a mirror image duplex that has been unoccupied for years (I think nearly 15 years).  

There was no smell of sewage and water was clear, and but my realtor suspected some sort of backup rather than a burst pipe bc we didn't see a burst pipe anywhere.

If it is set up that the client receives membership interest or shares in an entity that owns real estate then that would not qualify.The exception (sort of) to this are specifically syndicated products that are sold as securities and set up so that client does receive a tenant in common interest in the underlying real estate asset.  

Let them sort it out and stay out of the middle of it.Now whenever a tenant doesn't take my hint about not working on the property.

As long as you have buyer representation you would be due some sort of commission I would think...We have programs like that here in DC.

Do you have to sort of "join" them before they show you anything?

In all honesty and joking aside she is the sort of tenant that you need to "stick to the book" on.