Hi Willie,
A good place to look is here:
https://www.rossvideo.com/graphics-system/xpression/hardware/#prime (it says Prime on the anchor, but Designer uses the same hardware). I'd say that these are the *minimum* specs (not in terms of power, but age), as the CPU listed is an old Broadwell part, which is required for Windows 7 (newer CPUs don't support Windows 7). As XPression 7 runs on Windows 10, you can get newer, more widely available hardware. However, I think you need to stick to nVidia graphics.
I recently did an upgrade of our ageing XPression Prime system. As it has version 6.5, which requires Windows 7, we followed the specs listed on that page very closely. The only difference was the CPU as I couldn't get the exact part they listed (everyone was out of stock). I got the i7 6800K instead.
James.
#XPression