First time poster, fan of the company. I was curious if it would be a good idea to grab one of the older mainboards that’s currently on sale such as the 11th gen intel boards for a proxmox server I want to build just to test and experiment with. I figured grabbing that and the cooler master case would be cheaper than building a new machine.
Nothing would be mission critical stuff, just small docker containers and maybe a VM or two to experiment in. I’ve never used framework stuff before nor proxmox but I want to experiment with the hardware and software. I have made desktops before, so I do have technical knowledge.
Is there any recommendations or things I should consider with this build.
I have several server-style systems, handling Network-Attached Storage, NextCloud, Home Assistant, PiHole, a web and email server, and other services. None of those needed a lot of CPU power, so I bought really inexpensive parts for them, and each system handles multiple services. A couple of those systems are Raspberry Pi 4’s that I put together for less than $100 Canadian apiece. So you could almost certainly build a “test and experiment with” machine for significantly less than what a Framework motherboard, case, and various other necessary pieces (memory, storage, power supply, expansion cards, maybe wifi card and antenna, etc) would cost you.
That said, if you have the money for the Framework, I’d recommend it instead. Those cheap systems were cheap for a reason: they’re all very low-powered, CPU- and memory-wise, and of limited general use. You can run lxc-based VMs on a Raspberry Pi 4 with only 4GB of memory (one of mine runs two special-purpose VMs, as well as serving as my network router and firewall, all without GUIs), but they might be too limited to do much.
I have like 2 RPi 2s I’d like to stock up on 4s in the future. I used to use like a 4th gen intel nuc as my little test bed small sized PC but I’d like to retire it and swap to something more modern that’s also small and I also would like to play around with a framework board along with newer software.
One of my workhorse servers is a sixth-generation Intel NUC. 
I was really disappointed when I realized that Intel wanted more expensive and higher-powered machines, and had discontinued the extremely useful lower-powered ones.
The Framework motherboard is certainly a good candidate for that use-case.