Where are the potential challenges? Are there any aspects to network performance management implementation that might pose problems for me or my client?
The biggest issue is ensuring that the technology you select solves the client's problem. Clients don't always know what they mean when they say something, and you need to be sure that you grasp the underlying problem or need and proceed to correct the corresponding thing.
Another potential problem is working with a technology that you're not completely familiar with as a solution provider. Do your due diligence -- test and understand the product prior to actually deploying it. Providers sometimes get into trouble when they assume everything works the same way, implementing a device, an appliance or some other solution without ever trying it. It's not enough to read the product features on the box or study a product sales brochure. Know how it works, know how to deploy it, and verify that it will map to the client's needs.