Depends on how you want to do it.
Option 1:
Get the storage system sorted, then just transfer it to the new system (assumes it won't require Herculean efforts in terms of locating and installing adapters and such to fit the new internals if applicable). Thus making the system change that much faster/easier (also assumes the boot volume is not part of the primary storage volume, which IIRC, you don't do - OS is separate).
Option 2:
Put in all of the time at once, and upgrade to a new system and storage re-configuration simultaneously.
Both have their merits, but Option #1 allows you to space out both your time and budget (i.e. storage system upgrades + new system = OUCH at the level you're using IIRC).