First Time Home Buyer Help

Hi, I want to purchase a single family home. My only problem is the down payment. Recently, my parents offered to loan me up to 150k out of their home equity line to fund the down payment. How would that work when applying for a mortgage? Also I am not sure how a HELOC would work, if my parents write me a check for 150k, would that be considered a gift or a loan? Also would it be advisable to borrow as much from their HELOC as possible since the interest rates are lower than a Mortgage? Do HELOCs usually get paid off by a monthly payment of principal + interest, much like a mortgage? I am looking for a 400-450k house and make over 120k a year and spend very frugally. Any advice?