How do I structure a fix & flip (rehab deal) while the owner holds the mortgage?

[Michael Henry]

I have a deal in Milwaukee, WI where I can partner with the Seller to rehab his home and keep the proceeds over a dollar amount. The owner has a 1st Mortgage; a good ARV is there and the numbers work. What I'm wondering is how do I protect myself (My Money) during the rehab and afterward while the house is on the market?

Should I do a lease option and language within the option that protects me?

or

Should I take title to the property (somehow)?

or 

Other suggestions...