#1. There’s a recent thread on ripping which you might find useful:-
#2. I have approximately 2500 CDs ripped to flac which consume around 700GB so 4TB will be more than sufficient.
#3. You will need a DAC to provide outputs to your system but it’s worth doing some reading up on Roon Ready & Roon tested devices then post specific questions here where you’ll get all the help you need.