My thought at this point is to partition the M2 SSD in two with half going as the C drive, (programs, scratch disc, etc.) the other half for storage. A 2nd M2 SSD partitioned for a raid and extra storage. As I stated before, in my current build I have 1 SSD (120GB) and a pair of 7200rpm disc drives.
Not sure what you're using a RAID for here, seems unnecessary to me.
As of today, my thoughts are to copy the contents of my internal drive (desk top folders, programs and files) to external drives so that I can retrieve them after the build is finished.
I just pull the old SSD and keep it as a back up. Then a clean install onto the new drive. Although a full re-install is a bit of a pain, it's always nice to start from scratch again and looses any clutter. Of course you need to ensure settings, add-ins, plug-ins, macros, actions etc are all backed up and easy to put back. I do this when I build a new system as a way for ensuring my back up policies work ;-)
FWIW this is how my system is set up
C: 1tb M2 -- System and programs
D: 4tb 7.2k HDD -- Photos
E: 3tb 7.2k HDD -- Video, sound recs etc
F: 1tb SSD LR catalogues, ACR cache, C1 catalogue
Pretty simple and easy to maintain.