This is not a system I can "grow into", it must be capable of this performance from the very beginning. I would suggest you load test extensively and make load

This is not a system I can "grow into", it must be capable of this performance from the very beginning. I would suggest you load test extensively and make load testing a part of your development process from the get-go. Initially, I would load test to evaluate approaches and estimate hardware needs. As you develop, I would use load test each version and ultimately simulate a real world scenario before letting it loose. Rather than speculating, you can actually know what approaches work best, and have some guarantee the live system will work as required.