There's not a bundle right now so you have to order it piecemeal. The SSD part number you typed should be ASA5500X-SSD120=. You only need to order the off-box PRSM if you don't want to use the on-box variant. I do recommend using off-box where possible as it makes it much easier to keep the NGFW (CX) configurations synchronized.
Your base ASA needs to have current Smartnet support to add on the NGFW service subscrptions.
I haven't had the occasion to rebuild a RAID model, but according to the last line of the installation guide, "If you replaced the drives in the ASA 5545-X and ASA 5555-X, the ASA rebuilds the data on the SSD because it is configured to be in RAID1." You will need to partition and install both the boot image and system image when installing a new SSD.
