Hey Skrumpy I'm in a similar boat as you, but I don't have the experience yet and my needs are lower. All I'm hoping to accomplish is a virtual 1080p light gaming (Dark Souls Remastered at the most) machine for myself, a virtual 1080p light gaming (WoW at the most) machine for my wife, a basic home media storage solution for my family with mobile streaming options (to tablet, phone, laptop, etc), and preferably a virtualized pfSense router on top of that. Bonus points for remote access options for management purposes. Both virtual machine peripherals would be in the same room as the server, not but 5 feet away.
I was planning on recycling some parts from my current daily driver build, and this PCPartPicker list (or similar) is what I've come up with. But the reality, similar to you, is that I don't know how beefy these things need to be. Does a Ryzen 7 2700 provide enough cores at sufficient speeds? How much RAM does a virtual light use (but still gaming capable) machine really need and at what speeds? Are there limitations on which GPUs can be passed through? How much utility bill benefit would I really observe going with a low TDP CPU vs say a Threadripper?
It's surprisingly hard to find people having done this in groups with discussions to peruse, unless I'm looking in all the wrong places. I agree with you that this should be fully accessible even on lower budgets today with the hardware capabilities we're seeing and the price drops we're enjoying (aside from RAM).