You can check here how much of your workload is cacheable and what's the optimal cache size for your existing workload
And you can run it again after adding the SSD's to see if you should add more.
is not a big fan of hybrid systems (didn't tested NetApp one) i think you better start without. see if there any problems, what the out put of the above command (perhaps you don't need it and flashcache doing it's job) and of course see if the clients are happy and if you see latency anywhere , if there's no issues i would use the SSD not as a tier - but as a separate aggregate to accommodate workloads that really benefit from SSD (for example SQL tempDB). but that just an opinion - without knowing your workloads......