The purchasing agent may:

1. (a) Purchase for the county and its offices all materials, supplies, furnishings, equipment, livestock, and other personal property.
2. (b) Rent for the county and its offices furnishings, equipment, and livestock.

3. (c) Contract for services pursuant to this article and contract for public works projects pursuant to Article 3.5 (commencing with Section 20120) of Chapter 1 of Part 3 of Division 2 of the Public Contract Code.

   The purchasing agent shall make purchases, rentals and contracts only upon proper written requisition.

   For purposes of this article, the population of a county shall be the most recent estimate determined by the Demographic Research Unit of the Department of Finance.