Given the cost of the IQ4-150, I wouldn't skimp on the computer... Unless you have somebody in house who can troubleshoot a homemade computer as fast as the best tech support people from a workstation manufacturer (and without taking you away from photography at the day rate to support the IQ4-150), I'd call HP and get one of their Z-series workstations, or some smaller manufacturer with excellent service (as I recall, lots of people like Puget Systems, but there are others) and get one of their higher-end photography-focused systems...
Here's a nearly $5000 photographer's dream machine (taking into account that C1 likes graphics cards) using a 9900K from Puget Systems
Link to system here.
And here's another one, this time with a 16-core Intel HEDT processor for about $1000 more
Link to system here.
And here we are with a Threadripper, for those who prefer AMD
Link to system hereAll of these have:
64 GB RAM
A GeForce 2080 RTX, since C1 uses GPU power, and can use all the help it can get with that 150 MP back
A 2 TB Samsung PCIe SSD (that's the only storage on them other than the DVD drive Puget includes as standard, since images
that huge are going on an external RAID of some sort - the very fast Samsung is for Windows, applications and the C1 library/project files and previews. You could certainly dedicate that drive to C1 files alone, and add a similar 512 GB drive for Windows and applications...
You said no Macs, but I'd consider an iMac Pro (or the new Mac Pro if and when it appears).
HP's workstation line would be a great choice, probably some configuration of the Z4, but it means much more expensive Quadro or FirePro graphics - at least in their online configuration wizard, they won't let you pick a GeForce or a Radeon Vega (I don't think C1 cares between the consumer and pro cards) - if it does, that makes the extra money in the HP's graphics setup worth it (the same graphics performance will be a couple of thousand more in Quadro guise)...