Class 4 Fusion is built using C and on top of sofia-sip stack. It is extremely efficient so you don't need a lot of hardware power to run a high volume of traffic.
The size of disk depends on CPS and how many days of CDRs you want to keep in DB and in local disk. As you can push CDRs to Google Cloud, you don't need to keep many CDRs in local disk in raw format. However, you may want to keep more CDRs in DB for UI query.
You should estimate about 200 GB of disk space per day of CDR.