Thanks for your input. As it happens there are plenty of SuperMicro CSE-829U X10DRU-i+ 2U 12x 3.5" (LFF) to configure on that same site and they are cheaper. I have priced one up for £344.40 (without the card).

I’ve got 4 options for the chassis, which one would be most suitable (all the same price):

• 3x PCIe-x16 FH, 1x PCIe-x8 LP
• 2x GPU Support Cage - 3x PCIe-x16 FH, 1x PCIe-x8 LP
• 4 PCIe-x16 FH, 1x PCIe-x8 LP
• 3x PCIe-x16 FH, 1x PCIe-x8 LP, 1x PCIe-x16 FH

Then I’ve gone for:

• 1 x Intel Xeon E5-2695 V4 - 18-Core 36-Threads 2.10GHz (3.30GHz Boost, 45MB Cache, 120W TDP)
• 1 x SuperMicro Heatsink
• 1 x 32GB - DDR4 2400MHz (PC4-19200, 2Rx4)
• 1 x SuperMicro LSI-9361-8i 1GB (SAS/SATA) RAID Kit - 0/1/5/6/10/50/60
• 12 x Supermicro LFF Hot-Swap Caddy
• 1 x SuperMicro PWS-1K02A-1R ‘Titanium’ Hot-Swap PSU 1000W

Any suggestions on the GPU would be welcome, I am guessing something like a Nvidia Quadro P4000 could work? (data sheet)