Final answer:
For purchasing a complex and costly IT system, a consulting firm's report is the most beneficial since it offers a professional and comprehensive assessment, which is crucial for making an informed decision.
Step-by-step explanation:
When purchasing a complex and extremely costly IT system, the most useful aid would likely be a consulting firm's report. Given the substantial investment and potential impact on the organization's operations, a detailed and professional assessment is invaluable. A consulting firm specializes in evaluating such systems and can provide a comprehensive report that assesses the IT system's suitability to your specific needs, the total cost of ownership, and the potential return on investment.
While customer reference calls and peer recommendations can be beneficial for firsthand experience and testimony, they may not cover all the technical and business aspects required for a thorough evaluation. A site visit can also be helpful, but it might be inadequate alone for evaluating the IT system's fit with broader organizational strategy and needs.
Therefore, based on cost-benefit analysis, leveraging the expertise of a consulting firm through their report would be advisable as it can help mitigate the risk of imperfect information which is relatively high in the case of such complex purchases.