In my experience, defaulting on a student loan is real cause for a mortgage denial. If you know you are in default, why not contact the lender and ask to be put in forebearance. That will allow you to start paying back at a rate that you can currently afford. Once you have established yourself with a good payment history for 6 months - 1 year, you will be eligible for consideration to be approved for a mortgage.
Please remember also, that you need to have some good credit on your file. That means active things that you pay for monthly and pay for on time....for example, your car payment, charge bills, etc. If you don't have that type of credit but you have rental history, car insurance, health insurance, etc., that you pay monthly, you may be able to have alternative credit considered. Yours will be a complicated case but not impossible.
If you need any further assistance, please don't hesitate to contact me directly.