I don’t think there is any WordPress specific solution, but I personally use blitz.io to do the load testing for our solutions.
Actually you could also use one of the usual (free) tools (like Apache bench), but then you’ll have to consider a lot of other variables to make sure that you really create real-world-numbers (i.e make sure that you don’t have any other bottleneck inside your test setup).
That’s why I prefer cloud based load testing (there are also some other providers offering the same). 10 basic tests per month are free of charge with them. You just say how many concurrent users you want to test with (from 1 to 10.000) and you’ll know pretty fast if your stack is able to survice that. 😉
Also if you calculate average requests per second from 500,000 requests per month the number will be fairly low (and unreliable – as load is not evenly distributed). What you really want to find out is the highest number of concurrent users / requests per seconds you’ll have at the maximum peak time.