Procurement is a critical step in the setup of systems, as reverting decisions made at this point is typically time-consuming and costly. Especially Artificial Intelligence (AI) based systems face many challenges, starting with unclear and unknown side parameters at design time of the systems, changing ecosystems and regulations, as well as problems of overselling capabilities of systems by vendors. Furthermore, the AI Act puts forth a great deal of additional requirements for operators of critical AI systems, like risk management and transparency measures, thus making procurement even more complex. In addition, the number of providers of AI systems is drastically increasing. In this paper we provide guidelines for the procurement of AI based systems that support the decision maker in identifying the key elements for the procurement of secure AI systems, depending on the respective technical and regulatory environment. Furthermore, we provide additional resources for utilizing these guidelines in practical procurement.