The primary reasons for outsourcing internal audit activities typically include accessing a wider range of skills and competencies, complementing existing expertise for specific engagements, and strengthening core audit functions. Contingency planning, while beneficial, is not a primary reason for outsourcing internal audit activities as it pertains more to risk management strategies rather than the core objectives of outsourcing. Reference: = IIA's Practice Advisory 1210.A1-1: Obtaining External Service Providers to Support or Complement the Internal Audit Activity.

For a consulting engagement, planning typically occurs after the engagement objectives and scope have already been determined. In consulting engagements, the objectives and scope are usually agreed upon with the client at the outset, and planning activities then focus on how to achieve these objectives within the defined scope.
:
IIA Standards: 2010 - Planning
IIA Practice Guide: Consulting Services

According to IIA guidance, the internal audit plan should be based on an assessment of risks to the organization (1), designed to determine the effectiveness of the organization's risk management process (2), and aligned with the organization's goals (4). The development of the audit plan is typically the responsibility of the chief audit executive, often with input from senior management and the audit committee, rather than being solely developed by senior management (3). Reference: IIA Standard 2010 - Planning, IIA Practice Guide - Developing the Internal Audit Plan

In a mature control environment with limited internal audit resources, internal auditors should focus their testing on preventive key controls. Preventive controls are designed to stop errors or irregularities before they occur, making them crucial for maintaining control effectiveness. Key controls are the most important controls in mitigating risks to an acceptable level. By focusing on these, internal auditors can ensure that the most critical risks are managed effectively despite limited resources. References:
* The IIA's International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 2320 - Analysis and Evaluation.
* The IIA's Practice Guide on Assessing the Adequacy of Control Processes.