ThinkPad P70 Mobile Workstation (http://shop.lenovo.com/us/en/laptops/?menu-id=explore_all_laptops#facet-1=2)
I know I know it runs Windows but for $3500 you get and you can further customize if from here with other processors.
Processor: Intel Xeon E3-1505M v5 Processor (8MB Cache, up to 3.70GHz)
Display:17.3 4K(3840x2160) IPS Non-Touch Graphic Card (Yes 17" 4K)
NVIDIA Quadro M5000M 8GB
Memory: 32GB(16+16) DDR4 2133MHz ECC SoDIMM (upgradeable to 64mb)
Solid State Drive:
512GB SSD PCIe-NVMe (can add a secondary drive ssd or HDD)
Optical Drive / Secondary Hard Disk Drive:
I'm just sayin! Why can't apple just throw in some better internals.