We are talking <$15-20 range or cheaper. 

I have a unique opportunity to buy a property from a family friend for a discounted price that my Dad (recently divorced) will then live in and make the payments on a 10 year mortgage.

Renovated and designed with a unique finish What was the outcome?

These are to be taken with a grain of salt as they have not closed yet - but it can give you a fairly good idea if you are in the right range. 

My idea is buy something that can be fixed up a little bit to add some value, but nothing crazy.All the houses in my price range are only about $350-450/month for the mortgage.

I am also struggling with valuation as it is a bit of a unique property. 

@Jesse Paulsen There are currently 5 multifamily units for sale in FTN Hills, ranging from $433k-$675k, 4-duplexes and 1-4 plex.There are roughly 233 multifamily units in FTN Hills, perhaps you can send yellow letters to the owners to see if any want to sell to you

Use those 3 to get a $/sq. ft. price range, then compare to a space I'm looking to buy. 

Do you have rental comps with a $ per sq. ft. range?

As @Chuck Kramer mentioned, there are a ton of 4-7 bedroom high end new construction properties being finished this year and next, so you'll want to make sure that the aesthetic of yours will be unique enough to be competitive when all of those start entering the rental market.