now using the macpro just as a sample player is - phew - cool ;-)
having the samples on a separate drive at comparable access will IMO require 2 flash-drives in a raid-0 via thunderbolt.
USB3 has 5 Gbit/s theoretical throughput, thunderbolt2 has 20 Gbit/s, the fastest SSDs deliver ~600 MB/s (usually *only* 480 - 500 MB/s), which means a SSD raid-0 will deliver more than even USB3 is able to transport (1 Byte = 8 bit + overhead).
it doesn't matter at all to have OS and samples on one drive - after the OS (+ applications) has started there is almost no more drive access from the OS, everything is handled in RAM. some access could happen for the OS' pagefile (which also resides on disk), but to my knowledge this is not the case with VEPRO or MIR (once the arrangement is loaded).
how many cores are required/reasonable ... hmmm, besides that will strongly depend on your arrangement and amount of plug-ins unfortunately i currently don't have any performance figures for the Xeon E5 generation at my hands.