Here's a post where I'll place resources from the past two projects we worked on.

Models: MachinesModels.zip
Screenshots: MachinesScreenshots.zip
List of Machines: List of Machines.xlsx
OpenMachines Repository (SpringRTS Version): https://sourceforge.net/projects/openmachines/
Machines Legacy Repository (Unreal 4 Version): https://github.com/OpenMachines/machines