It’s currently vacant and we are considering selling, but the foundation will be an issue.  

The best thing you can do to prevent future foundation issues is installing gutters around the property to repel away from the foundation.

Hello I wanted to know what does it cost to do foundation & framing for a 1,500sq ft house being built in Lehigh Acres Florida. 

Hi Steven,It sounds like you and your wife, Renee, have a wealth of experience in the real estate and construction fields, which is a great foundation for starting your real estate investing business.Given your background in residential remodeling and home inspections, and Renee's experience as a real estate agent, you're off to a strong start.

There are companies such as Cover that offers a full service ADU deliverable from design through construction completion.

Does anyone have information if placing it on a permanent foundation will make sense from a long-term value standpoint?

I have my own designs and after uploading them and sending them out for a few rounds of marketing I eventually received some un-deliverables and they were barely legible along with their being major defects during printing.

Anybody have any experience with a heaving foundation in the center of a pier and beam house?   

Make sure its on a permanent foundation, has concrete pillars, skirting and axles removed and in some states require hurricane Tie downs. 

I have been told by a contractor that the foundation of the building is intact, so it does not need to be torn down but requires a "gut renovation down to the studs."