This study develops an integrated multi-criteria decision-making framework for evaluating manufacturers of steel structures for long-term business cooperation. The research addresses the growing importance of supplier reliability under conditions of increasing technological complexity, higher quality requirements and pressure to reduce production costs. The assessment was conducted in an SME operating in the steel structure sector. Thirteen experts evaluated seven manufacturers using technical, quality-related, economic and organisational criteria. The Best–Worst Method was used to determine the weights of the main criteria and their sub-criteria. Fuzzy TOPSIS was then applied to rank the manufacturers according to their distance from the positive and negative ideal solutions. The VIKOR method was used as a comparative technique to verify the consistency of the ranking. The robustness of the results was additionally examined through One-at-a-Time sensitivity analysis, Monte Carlo simulation and an equal-weight stress test. The results support the segmentation and prioritisation of manufacturers. The proposed framework improves the transparency, consistency and repeatability of supplier evaluation. In addition, the framework enables the identification of indirect operational costs associated with incomplete documentation, ineffective revision management and communication errors. Given the application-oriented nature of the study, further research should verify the framework using larger samples of enterprises and in other industrial contexts. The proposed research procedure may be adapted to other industrial enterprises by modifying the criteria hierarchy and weighting structure.