It's going to be hard to dive in without wanting to relocate yourself and limited funds flying solo.

For ADU's - I've seen older posts on them being attributed limited value despite the additional sqft, bedrooms, bathrooms, etc. they add to the property, but the methodology seems to be geography specific.

I have thought about making him just a limited partner and telling him he will get a 8% return yearly on his money.

The loan loan limit for FHA in Richland County for a 2 unit is $538,650, I am not super familiar with the market in Columbia but with a quick look on Zillow I saw a couple of Duplex's well within that price range.Best of Luck!

For instance I know of a portfolio program that allows a non-occupying cosigner on a duplex with 10% down offered in a limited footprint of states.

However, if there are limited options to begin with, narrowing that criteria may make that "perfect fit" even harder to find.

Because that has a HARD time limit on it.

They can provide references for just about everything you need, including but not limited to: lawyers, property management, lenders, contractors and more. 

In the liberal California county I live, if you have rented an illegal unit to someone, they can go after you for all rent they have paid you(no time limit) as well as relocation costs. 

Just remember each Pearson has his own limitations and life perspective.