MysticalGnome

I use PrimeOS on my Dell Latitude 2in1 which works great however not all hardware is supported, notably wired connectivity and webcams.

Most of this should be more than doable on a normal PC, an 8-core i9 9900K or maybe the upcoming Ryzen 7 3800X would probably be fine I think. I would go with at least 16GB RAM and maybe more if you want to run them simultaneous. I also think you should be able to play some older games on it while the game servers are running but this depends on the titles you want top run. A more important thing I think would be network connectivity, if you are going to host a steam cache and you have more than 6 PC's downloading at the same time i would opt for a 10Gb NIC and a switch with one 10Gb port and the rest of the ports Gigabit this would greatly increase throughput and prevent latency during gaming because somebody is clogging the line when they upload files to your share. As for the OS, Windows 8 or higher would be fine although some old games may not run on it. I take it you found this already but I'll add it none the less:

What kind of CPU are you using right now? Also can you spin up a second host to see if there is an issue with the guest OS itself?

This looks to me that you either don't have internet access on that system or that DNS isn't functioning properly. You could try manually setting the DNS server to 1.1.1.1 or 8.8.8.8 in the network manager.

What kind of system are you running it on? how much cpu cores did you assign to your VM? (4 would be the bare minimal for any gaming) Did you install the vmware tools on your VM.

I am not familiar with MailCow but there are a number of tutorials that help you setup a basic mail server on Ubuntu using Postfix, digitalocean has quiet a few they are pretty basic but they get the job done also I wouldn't use POP3 go for IMAP. But this does not cover things like creating the A and MX DNS records and things like preventing your mail server from becoming an open relay server not to mention spam filtering or blocking account hacking attempts on your mail server. As for connecting it to cloudflare a quick google will tell you how to do that. You could also just host your mail domain at a e-mail host provider, it's very affordable and takes away all the complexities of setting it up yourself and maintaining it.

Most likely there is no password on the user and it can only be accessed with a ssh public key, if you can login to the machine with putty as the ubuntu user you could get root access with "sudo -i" and than assign a password to the user you want to access "passwd <insert_username>".

Also the USG from Ubiquity is not especially powerful so I would opt for something more high-end if you want higher speed connectivity. My personal experience with these types of VPN services is that it can drastically reduce the snappiness of your internet, this is mostly because the outward facing side of the VPN is limited in available bandwidth and will be slower than your internet speed unless you already have some really slow internet.

Your question is somewhat vague, from what exactly do you want to protect your machines?

You could just portforward it from your router to your server, but this might not be the most safe option. Another way would be to setup a VPN server on your home network and make it accessible from the internet so you can create a secure connection to your home network and have access to your server from there.

The installation is already complete at that point, GRUB is the last thing to be installed but you do need it or you won't be able to boot into your OS. I would suggest hooking up your laptop to an external display, it should work for your Ubuntu installation and possibly your bios as well.

Anything that is supported by DOS 6/7 really, so a 486DX or Pentium 1 system with a sound blaster compatible sound card. 64MB of system RAM should be more than enough to run anything in DOS, a floppy drive and an IDE CD-ROM drive should also be on the list. With that you should have everything you need. You could go the easier route and get a Pentium 2/3 with Windows 98 which is essentially DOS with a GUI.

HP DL360 G8/9 would do just fine they only take up 1U in stead of 2U and perform the same, Dell PE R610 is also a good choice. We used XenServer in the past and pooled several servers together with a SAN backend which worked great as it allowed us to live migrate VM's to another server so we could perform maintenance on the Xen host, we also use BIND and while maybe not the best DNS server perse it does perform great, the only thing I really don't like about it is that it is really easy to screw up the configuration with a typo.

You can buy them no problem, like here. There are multiple models available with 2 or 4 GB of RAM, the APU4 seems to be the latest iteration. There is also a case available for this board in multiple colors for a few euros.

Another low power (5 watts) option could be the PC Engines APU 3 B4 it's a great board which has 3 Intel NICS on it and 3 mini pcie slots, one of them doubles as a msata slot for your OS. It also has two sim slots for LTE. Only downside is that it has no video output so it is strictly terminal and serial connectivity.