Hello, Midimusik
I'm not sure if I could recommend regular hard drives for running libraries. The current ones seem to operate, and are optimized, to be run through SSD's, so they should benefit from that. Personally, I'm using a PCIe card Asus Hyper M.2 x16 card, that holds up to 4 PCI M.2 fast drives. Something like that could be built on a small form factor case.
Besides the system drive (a 128Gb ssd) I still have two other 500Gb ssd drives for the older libraries that doesn't require multi-mics.
If Intel is not mandatory, I would recomend you to check the AMD Ryzen platform, although it should be best first to check the cost/benefit factor. Both should handle the 128Gb RAM you're looking for (on Ryzen that's possible from 2nd Gen - the 3000 series - and more recent). The only thing you'll need to get on most Ryzen cpu's is a dedicated graphics card, though. Then again, any Geforce 710GT with 2Gb is more than enough to handle the requirements, without spending too much.
Considering the current processing power of modern cpu's, perhaps you could even get things done without needing a third pc.
Hope this info could help you out.