when negotiating a rent with the option to buy lease?

I would like to buy this house in 1 year. Normally a portion of the rent will go towards the down payment but it seems like we would have to give extra than the original rent stated which was a little more than I was thinking of paying right now.

what other questions would you ask? what if the house is not worth what it is selling for now within 1 year?

any suggestions/opinions/expertise would be greatly appreciated.