This is a legal question, more than a tax question. Depending on the laws of your state, if the owner still has a mortgage on the property they can not sell it to you under a rent-to-own contract without the written permission of the lender. Even then, the wording in the contract means everything to all parties involved. You need the legal advice of a real estate professional, more than a tax professional right now.
From a tax standpoint in order for you to deduct things like property taxes and mortgage interest, two criteria must be met.
1) You must be legally obligated to pay it.
2) You must actually pay it.
Right now with the mortgage lender, you are not legally obligated to pay property taxes and mortgage interest. So unless you have a legally binding agreement that *clearly* spells out what specific expenses you are required to pay, this could become a problem down the road.
In my experience, 100% of rent-to-own contracts never results in the renter actually owning the place when all is said and done.