To address your question about child support / alimony, please note that lenders require documentation, usually a recorded judgment, to verify the amount of the payments and the payment duration must be three years or longer for the lender to include that income in your mortgage application. As long as you have proper documentation and the child support / alimony payments will last at least three years, monthly income from child support / alimony can help you qualify for a mortgage.
The answer to your question about what size mortgage you qualify for depends on your monthly gross income and debt expenses as well as your financial profile including your credit score and job history. You can use our Mortgage Qualification Calculator to determine what size mortgage you can afford based on different monthly gross income (including child support) and debt expense figures. The amount of the mortgage you qualify for plus your down payment equals the price of home you can afford.