I have a property in Memphis, TN that is zoned as mixed-use (RW).

The property in question is a mixed-use, with a ground floor commercial unit, and one residential unit upstairs. 

And get a detailed report on their letter-head. 

Mobile lenders also do not seem to want to lend on it due to the mixed nature of it.

When I'm preparing these letters/documents, I'm really learning as I go.

There is no set mix %.  

I’m in the process of signing an offer letter for a home in the Lake Carolina area of Columbia, SC, listed at $399K.

Some investors are leaning on smaller units and even including some mix use options in their search as well to see if those numbers make more sense. 

Most people will tell you that your second choice can't be done, but the IRS has issued two (2) Private Letter Rulings on point where they did allow the taxpayer to build on property they already owned through a 1031 Exchange. 

I would then follow that up with a lease violation letter aka notice to correct.