It is hereby declared that congested, insanitary and dangerous housing conditions which exist in portions of the cities of the first class in this state are a menace to the health, safety, morals and welfare of the public; that the correction of these conditions is essential to the general public health and safety; that it is only practicable to accomplish that purpose by the investment of private funds for the acquisition at fair prices of the lands and buildings upon and within which the conditions obtain for the demolition of the insanitary and unsafe buildings and the construction of new housing facilities under public supervision in accordance with proper standards of sanitation and safety and at a cost which will permit rentals within the means of wage-earners of the lowest earning capacity.