Will there be any network (or other) downtime deploying or maintaining the performance management project?
It really depends on the implementation that you intend to deploy for the client. As you might expect, high-availability solutions are typically quite expensive. If a client's need is specific and targeted, and the return on their investment is expected to be small, they may be willing to accommodate some amount of downtime in exchange for low cost -- that ties back to the cost and budget issue. For example, appliances tend to cost a bit more than software-based products, but generally impose very little downtime if I've done my job right and tested and staged the appliance properly prior to deployment. Then it's a matter of disconnecting the right cables and inserting the appliance in a few moments of downtime while the traffic gets rerouted.