Put in a pair of 3 GHz quad-cores for a start.Anything that would increase performance would do.
You'd have to use a riser to separate the GPUs from the PCIe slots and come up with some kind of mount for them. If you can get a Thunderbolt 2 PCIe card to fully work, you might even be able to use eGPUs.I was wondering if the space could be rigged up also with suitable GPU's for rendering.
Nope.Remember that SSD's are actually smaller than 3.5".