Answer
I think it's always important to involve multiple stakeholders when evaluating technology partners, not just the IT team. The people actually using the technology day-to-day will tell you things that no demo or sales pitch ever will.
When we were evaluating partners at our company, we involved finance, operations, and end users from the start. That's actually how we ended up working with Redington - they were the only ones who sat down with different departments separately to understand their specific pain points before proposing anything. Most others just spoke to the IT head and called it discovery.
A few things that genuinely matter in my experience - does the partner have deep expertise in the specific technologies your business depends on, not just surface level knowledge. Are they certified by the vendors they represent? And critically, what does their support structure look like post-implementation because that's where most partnerships either succeed or fall apart completely.
I'd also strongly suggest speaking to at least two or three of their existing clients in a similar industry before signing anything. References tell you far more than case studies that partners carefully curate for their website. Build your own partner evaluation framework and revisit it as your business needs evolve.