Answer
Each partner type answers a different question well and a different question poorly. A reseller-style partner is strong on licensing structure, hardware and software sourcing, and keeping a support contract current, but is rarely the right source for an honest opinion on whether your architecture is sound. Assessment-led consultants are worth paying for exactly that honest opinion, including where legacy RPG or COBOL code creates real risk and where it does not, but a pure assessment engagement will not staff the ongoing work. Managed service partners fill the day-to-day gap, running backups, PTF management, security monitoring, and job scheduling, but they are not a substitute for a modernization roadmap.
Before any buyer conversation, write down what the internal team can already do well, where the actual gap sits, and how long that gap has existed. A partner who pushes toward a bigger engagement before asking those questions is a signal worth noting. It is also reasonable to run more than one type of partner at once, for example a managed support provider for daily operations alongside a consulting engagement for a specific modernization phase, as long as ownership boundaries between them are documented in writing.