As products become more electronically complex and component lifecycles are shortened due to rapidly changing technologies, obsolescence management has become a challenging problem associated with sustainable products. One of the most common strategies to manage obsolescence is to purchase enough components to sustain the product over its life in operation. This strategy is known as lifetime buy. Purchasing components for the life of a product depends on satisfying demand, as well as maintaining the product in the context of its use within larger complex products or systems. Firms must also consider manufacturing systems and support systems that were built for the product. Component loss due to damage or expiration in storage and scrap during manufacturing also contribute to lifetime buy quantities. Product support systems, such as warranties and product replacement programs, can consume a portion of lifetime buy reserves and lead to unsatisfied demand from customers. This paper takes on optimizing lifetime buy decisions by investigating the myriad of factors impacting total costs. A taxonomy is put forward that provides the basis for a comprehensive decision support tool, capable of assisting lifetime buy quantity decisions over the product lifecycle.