You are not required a property in Portugal to apply for a D7 residency visa, however you will be required to submit proof of accommodation in Portugal. A rental agreement should typically suffice.
Should you wish to buy a home in Portugal, there are no minimum purchase price requirements, however it is expect that whatever property you rent in Portugal will be “fit for purpose”. In other words, you cannot rent a studio apartment in order to house a family of 5 people.