Should I buy my place?

We rent a condo from my Grandma. She just sold the other one and we live in the other. We have been paying her every month and we have some insurance on it and so does she. She said she will sell it to us but now wants a down payment after a year of renting it. She at first said she'd go back and apply our rent to a mortgage, but now I hear nothing of that. We would also have to pay taxes and she claims we can't afford those (my husband works two jobs and makes a good living. I work part time and go to school full time). Should we buy it or just wait? She's almost 90 already.