At the same time you will be, most likely, paying more than market rent and thus throwing your money away. Chances are if the owner is willing to sell you the house in the beginning, he would take an offer from you any time during the period that you occupy the house for rent. So if you eventually may want to buy the house, rent it and make an offer at a later date. Don't be squeamish. Ask and you shall receive.
Chances are the owner is well aware of the problem.
Many people won't look at a FSBO because they are afraid that they ultimately will not be able to buy the house without some legal problems developing. But all you have to do is bring your Realtor with you when you visit the house, or make an offer, and you will have the same protections you would have if you were buying from someone who had listed the house with a Realtor. Don't negotiate yourself into financial problems. Sometimes people will see a house that they are fond of. Know your limits and know that you will find what you need in that price range.