15-30ms latency seems reasonable for that hardware (P420 in RAID 10 with 4x4TB 7k SAS HDDs.)

Basically your single SAS disk can do ~150IOPS random, so worst case of 4k random reads you will get ~600KBps.

For the migration if it’s sequential (depending on filesystem layout), the performance can be different, up to maximum streaming performance of like 100MBs for large sequential reads.

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Then 2x for your RAID config.

Start with the cheapest suitable switch and go from there.

IMHO DDR3 is too old to be worth it unless you get the RAM for free