The 8 tb spinner will work just fine, 16 GB is fine until you reach 500k or higher tracks. Storage doesn’t really need to be at SSD speeds. When it comes to moving over to the new rig, follow the Migration guidelines and you should be fine. (Don’t get rid of the old Core until the new one is up and going, imho). Tower cases are just fine. If you have the space, the bigger the better for working inside them.
For reference, see my current Core: The Soul of a New Machine