An NVME SSD you can plug into the motherboard without needing any cables. It is smaller than a candy bar and about 5 times as fast as a regular SATA SSD and even about 25x faster than a mechanical HDD.
I recommend getting a Samsung 970 EVO M.2 SSD that come in different sizes.
Of course, NVME Drives are somewhat more expensive than HDDs or SATA SSDs and the best plan usually is to get both.
A smaller NVME SSD for active projects, apps, and the OS. And a large HDD for Backup and Archiving.
Read more: Workstation PC
The Computer-Case, of course, will not influence the performance of your CAD Build in any way. Well maybe in terms of air-flow, but that can usually be neglected as CAD Work tends not to make your CPU or GPU overheat all that fast.
There are lots of Cases out there in all kinds of Colors, Sizes and from all kinds of Brands.
There is not much you can do wrong here. Be sure to get a big enough case for your components, usually ATX Midi-Tower or bigger.
