You gather global futures market data only or with options as well? It's simple to have a rough idea how many GB of data you will have in a day then estimate the total written in a month / year. In your case I think you should use raid 1 instead so that you that you can replace the dead ssd easily. Large size ssd doesn't cost much compare to ram, also if you only gather futures market then write speed of high end ssd should be enough for your case.
I'm not sure how you store those market data, the storage method is import as well. Text file and DB makes a big difference in R/W speed and search performance.