More importantly, how much? Component seems to be omitted from the part list.
Plenty of passively cooled discrete GPUs as well. Nice build.
Why would it be risky? Samba is an incredibly robust piece of software, and most of the prerolled NAS distros like FreeNAS, OpenMediaVault, Openfiler, Rockstor and such even make existing AD/domain integration a fairly painless and smb.conf-editing-free affair. Even a general distro like OpenSuSE's YaST management tool makes Samba administration and AD/domain membership fairly simple.
If it's at the insistence of the client, well, that sucks.
No problem. He has a cracking machine going there!
You would have been far better off getting 3 equally sized SSDs and running them in RAID0. Modern operating systems, SATA controllers, and SSD firmwares are very very efficient at command queue sorting and optimising I/O. Infinitely more efficient than running software and assets and outputs, all with different throughput needs across 3 separate drives and having drives with idle bandwidth while some others choking at other times.
Anyway, that's done now.. But some continuous backup software (Acronis true image springs to mind) to duplicate the working drive to the backup drive would be a good idea, that way the software can maintain file versions and deleted files as changes are being made, that would take the grind away from your friend so he can focus on getting work done instead of juggling files all day.