OK, final remarks, now that it's all set up. The machine now has an i7-4790K CPU (4GHz base speed, quad core with H/T), with overclocking turned off and the fan setting to turbo (which makes it do what it should do in standard; not often ASUS get stuff wrong, but nobody's perfect). RAM 16GB, GPU GT-1030 2GB (motherboard GPU disabled), HDD: 1TB SSD.
The machine is now so fast that Pinta (Paint.Net clone for Linux) runs at a decent speed, despite there being no equivalent to
ngen, that I don't really need Paint.Net in a VM any more; so the only reason I have to run Windows is UDB; please Satan somebody make a better Windows Forms for mono

The city sandbox runs fine as does the first part of Elementalism, though I have yet to dive into the latter fully. This is fast enough that not only is UDB fine, I can even
play Planisphere 2 in the VM

Albeit at about the same speed as my roughly contemporary i3 machine
It helps that VMWare Workstation Player is free for personal use and is
a lot more polished than VirtualBox, particularly as regards virtualisation of the NVidia GPU.