Most mortgages are not transferable. You will most likely have to get a new mortgage in your and your husband's names. However, it cannot hurt to talk to your mother's lender.
If your mother wants to accept a below market price for the home, she will gift you the difference between fair market value and the price you agree to for the house. However, you have to be careful how you do this. Lenders often do not want the down payment to come from a gift. They want you to come up with the down payment yourself. The way around this is to have your mother gift you cash and then let a few months go by so when you lender asks to see your bank statements, they will not see a big deposit and you will not have to tell them where it came from.
There are limits to how much can given tax free. I believe the current limit is $13,000 per person. This means your mother could give you and your husband $13,000 each. So if the fair market value of the home is less than $166,000, you could make this work if she just wants to cover the outstanding mortgage balance of $140,000. If the fair market value is higher, then you might have a problem. She could give you $26,000 and then take a note for the rest and forgive up to $26,000 of the note next year. However, this process may impact your ability to qualify for a loan.
A real estate attorney can help you with the paperwork. It should not cost much, but lawyers charge a wide variety of rates. Do not be afraid to ask how much it will cost, but make sure you hire an attorney who specializes in real estate.