When designing a system for RStudio Workbench / RStudio Server, how much local or SAN storage would you recommend and do you have an I/O requirement?

The RStudio IDE has very few requirements on the storage side. If you are interested in leveraging load balancing/clustering, you will want a shared file system which many of our customers will implement using a SAN of some sort. The IO requirements on the storage layer are driven by how much reading/writing your code will do. Many data sets that are < 2GB will do fine with an entry-level SAN, and you could probably get away with NFS for that type of volume. However, it is difficult for us to provide recommendations on this front, and the best option will depend on your specific system and requirements.