Internet here is slow, so top notch ethernetcontrollers will just be as slow as a 20$ controller. However, is there anything special to look out for?
Just one that does what you need and is supported by your OS.
Can a VM be on the SSD and mount HDD raids? How do I even approach it? I hadn't got the time to properly analyse this aspect, but any input is greatly appreciated. Or do I have to do it the other way around and configure the RAID1 in Windows and just store the disk image in that RAID1 volume?
Yes, presumably you're going to use something like Windows Server 2019 or similar as your base OS right? So your SSD's are one drive and your HDD's are just another drive - your VM sees two or more virtual disks, one on one drive, the other on the other drive. So yeah just boot off the SSD (can you get a second for R1?) volume, and you can store your VM boot disk on that too, then have the VM's data drive on the R1 HDD volume.
The old system is running Ubuntu 14. Can it just be cloned into something a virtual environment can read?
Yes, often you can, google 'P2V' based on whatever hypervisor you want to use - often you can just copy a physical server's disks to a single bootable file.
Is the guest-OS (so the Linux VM) capable of reaching the machines connected to the switch? I mucked about with a VMBOX a few times and I always had trouble with connecting to Network Attached Equipment.
Should be, obviously needs a bit of config to make it work but you have a simple enough setup to have to emulate.
Windows 10 introduced a strong update policy (good thing); however, what can be done to configure the machine in such a way that upon reboot the VM automatically resumes and that no planned reboot happens when the VM is running.
You can stop Windows from doing automatic updates which will stop the auto-rebooting, and most hypervisors can be started automatically on boot and be configured to automatically start one or more VMs on start.
My experience with VM-system is limited to VirtualBox; is this a solid choice for the described setup?
If you need to boot from Windows then I'd be tempted to use something like VMware Workstation - personally I'd just run VMware's ESXi on the hardware but that's because I know it well - but it's more important that you know how to configure and manage the system, and if your skills are in Windows then that makes the most sense.