Sounds like you want to do a lease option.
Set up your basic rental lease with your renter for a year. Then set up a separate option agreement which gives your renter the option to buy the property at the end of the term at a set price and collect a fee for allowing them to lock in that purchase price (the fee can be whatever you want $1- $3000 depending maybe on the price of the house).
If at the end of the year they choose not to purchase the property they can continue on with a new rental lease or move out and you can find a new renter. They have the option and not the obligation to purchase with the option agreement.
And like you and others said do not count their rent towards the purchase price, that makes it way to complicated and has larger ramifications.
Not sure how to handle your other question of the renter bouncing out for 10 months and then coming back to purchase - I think if they do that they most likely have horrible finances and cannot qualify for a loan and actually follow through with the transaction.
Good Luck!