If her visa ends by end of Oct, now is too early to process contract , I am not sure about if you use agency or not . The earliest you can submit contract application is about 4 weeks before the visa expiry date . Anyways , since there is about 7-8 weeks after you interviewed her and confirmed to sign, there is too much time for her to back out even she said ok to sign with you .
When you start looking again , better choose someone whose visa only 1 to 1.5 month left, will be getting close for both of you to confirm ( but they can still back out and wait till the last 2 weeks after they are released from their employer to confirm someone they really want to sign )