If your father is going to borrow money at 5% to make money,you need to make sure your investment makes a guaranteed return of at least 9% or more (5% for the loan plus 4% inflation). Do a wiki search for “cash on cash”. Not a lot of people in the stock market are guaranteeing 9% returns nowadays. I really don’t recommend taking equity out of a house in order to invest in the stock market. Way too much risk.