They're enough for anything web based in my experience. I have a m710q Tiny as a server and I've been impressed by it so far. It's an older model than what you're looking at but even then it'd be enough for desktop usage. For future proofing they should be good for a long time with that CPU. They can be upgraded to 32gb of ram and have a slot (or two) for M.2 SSDs if you're needing a speed boost there.

If you're looking for speed, you won't find it with the m920q tiny unless they're equipped with SSDs. We use them at our company with mechanical hard drives and the biggest report of issues is speed, because the poor hard drives can't handle Windows 10.l, even for just checking emails and running Edge.

Also, make sure you upgrade them to 16GB RAM too.

They're probably fine with SSDs, but not with HDDs.

Edit: Just checked the listing you posted. Those ones should be fine as they have 16GB RAM and SSDs installed already

Not sure which instance I should post it to

The instances you speak of are actually called "communities". Instances are Lemmy servers that contain them. So lemmy.world is a instance, while !techsupport@lemmy.world is a community in the lemmy.world instance.

The desktop you suggested seem to be more than enough for very basic usage (browsing, presenting etc.). I do suggest installing Linux on them, unless you need to use Windows specific apps.