Intel/Win applications - NUC, MiniPC, Server. What do you recommend?

You don't need any extra server hardware. If you do proper backup, I don't see any problems.
Most likely the hard drive will fail, the rest runs for years.

Home Assistant on VirtualBox is a bit tricky. As long as you don't want to pass through USB sticks or similar, it's fine.

Intel NUC (possibly used) and then install Windows. No idea if there is Gira Project Assistant and ETS Professional for Unix!?

The OP has now perfectly solved his situation.
And by the way, ETS is not available for Linux.

Quick feedback:
I now have the HP Mini PC running in the rack and can easily mess around remotely in the ETS from all Macs, just as I wanted.

I just regret struggling with Bootcamp for the past few months and not coming up with the idea earlier...

Can you please briefly say exactly which one you chose with what configuration?

HP 800 G2 Thin Client with i5, 8GB RAM and 256 GB SSD

It does its job (Gigabit LAN)

It does its job (Gigabit LAN)

Nice piece. Unfortunately, only very limitedly expandable, so not really a replacement for me. Still, thanks. I think I have to wait until I have money left for a 19-inch server. I would like to replace my Windows Server 2016 Essentials with the existing Datacenter to virtualize in HyperV. Then I need more space for drives. A SAN would be a bit too much of a good thing xD

A cheap server (because it's mass-produced) would be, for example, a DL380 Gen8. It supports inexpensive DDR3 reg. ECC. 256GB in 16GB modules usually costs around 200 - 250€. A second bay for 16 drives is also available for a song. Suitable RAID controller Smart Array P420 with battery and 1GB is about 50€. 2x Octacore Xeon, lots of RAM, buy empty chassis or a few new 1.2TB HDDs (about 180€ each), RAID, redundant power supplies, RAILS etc. make it a cheap server for the datacenter. Use ILO license or standard remote functions, 10GBit card also cheap ... etc. They run fine. Only BIOS / firmware should be up to date because without a maintenance contract you can't access it anymore (without connections). Only electricity costs a few extra euros, but it stays within limits at a lot of idle or the server heats the basement ;) Additional (lots of) storage cheaply would be, for example, connectable in the form of a NAS via iSCSI ...

2 Xeons are too much for my purposes. Currently, as mentioned, an i5 3570K throttled down to 1.6 GHz is sufficient. If I knew that my Ryzen 1800X along with the Asus Crosshair VI and the 32GB RAM are suitable for Windows Server, I would probably wait until I replace my desktop and use these components as a server. If not, I am saving up for an HP or DELL single CPU.