Take a mortgage for out of state citizen

Hi, My wife she is a US citizen but she never lived in the US. All of her income generate out of state, but every year she also submit an annual report for the US as well. A few months ago we opened for her a bank account in Bank of America and got for her the first Secured credit card. She started to build her credit score (based of credit karma). We almost don't use this card. My question is, when she will have the option to ask for a mortgage in order to buy a rental property (for investment). What will the bank requirements in this case? Thanks