Rent to owns are rare, and even more rarely are they a good deal for the renter/buyer. You are just a tenant until you are able to qualify and complete the mortgage. Landlord/sellers love these because they collect up front non-refundable deposits and over market rent and rarely ever see them complete the sale.
A better option is to solve whatever issues are preventing you from buying now. Save your down payment, fix your credit and make a real purchase from a lot more choices when you are really ready.