For the engine, you will need at least 58 GB. I always use 62 just to be on the safe side.
If you use 50, your install will fail.
You don't need an ISO domain. ISO files can be stored in "data" or
"vmstore".
Each vm you create should have 2 disks, 1 for the o/s & 1 for the data.
The o/s disk goes in "vmstore", the data disk goes in "data".