1. An 18-core processor option, 4TB of storage (hopefully NVMe) and 128GB of RAM. That's crazy for an iMac form factor. I'm surprised Apple is even continuing the traditional Mac Pro form factor, considering.
2. Specific details on AMD Vega's pro models:
Radeon Pro Vega 56 graphics processor with 8GB of HBM2 memory | Configurable to Radeon Pro Vega 64 graphics processor with 16GB of HBM2 memory
Having a GPU with 16GB of HBM2 doesn't necessitate an external graphics card in the vast majority of cases. It'll be interesting if you can then hook up additional external graphics cards to augment its GPU power for either high-end 3D work or deep learning applications.
I'm guessing we may see people hooking up external Nvidia GPUs just so they can take advantage of CUDA, which is kind of a shame considering the specs of the onboard Vega hardware.
Metal 2 introduces compute shaders however, so I'm not sure if Apple intends to challenge CUDA primacy. Considering their reliance on AMD GPUs, it would be in their interest to do so—or at least somehow convince Nvidia to discontinue their anti-competitive practice of locking CUDA to Nvidia GPUs.
[0] https://www.apple.com/imac-pro/specs/
EDIT: As other commenters pointed out, I incorrectly speculated the Mac Pro form factor was done for. On the contrary, its continuation is officially confirmed. My comment has been updated accordingly. Thanks!