It's takes up very little space on your c drive. It's a small download. No data is stored on your C drive do you'll be fine.
You only need 1 parity drive. One drive for parity will allow you back up or rebuild recovery of any of your data drives in event of a data drive failed.
So perhaps one 3TB for parity. That's it. Keep all the drives you have now and pool them. You can add more storage drives to the pool anytime.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."