Selfhosted

I disagree with those saying that you can't do a build for that budget, but I would suggest looking into used parts, at least for some things, to improve the result significantly.

Since your system goal doesn't seem to be storage related, as nextcloud includes storage obviously, but typically isn't used to house multi-terabyte data sets. So assuming you can make that work for the "future homelab projects" to with dual 500gig NVME as storage. Search for a used mITX board+CPU that can accommodate that (has the slots), and go from there. Things like CPU cooler, if not part of a possible mainboard+CPU bundle, should be selected after the case at that is the limiting factor for it. Didn't skimp on RAM size if you can (new or used is fine, depends what you can get in your area).

With this list you're basically done to get it up and running.

I've got a ryzen 5600g, 750w EVGA 80 plus gold PSU, 32gb ram, 1tb ssd and 2tb HDD, in a 6 fan MSI case, MSI b450 gaming pro carbon max wifi MOBO, fully functioning. All parts are new if your interested.

Do you want a rack? I personally just use old desktop parts. It's currently a massive ATX box (old PC), but I'll be upgrading to ITX once I upgrade my PC again.

So my recommendation is:

1. Build around whatever you have on hand
2. Whatever is cheap and meets your current needs and can expand to meet future needs

You don't need high specs for most things, just make sure the CPU can do transcoding if that's what you're planning to do eventually. Consider N100 builds or things with laptop CPUs instead of a normal ITX build, that should save you some cash now and power usage later.

IMO MiniITX are a real PITA to build for on a budget. Most of the smaller components are sold at a premium because of their size.

I sell these things for a living and its exceptionally difficult to compete with pre-built ITX boards. Generally, I have to get a really great deal to come out on top vs some of the prefab models.

Because of that, unless you need something very specific and can't find it elsewhere, I generally suggest that you do some research and find a nice prefab one for your needs. If you don't mind spending the extra $, then building them is a hell of a lot of fun because you can customize them and you get exactly what you want, nothing extra.

Replacing the mini-rack with a completely 3D printable version will pretty significantly curtail the cost (between 1-300 euro because mini-racks are fucking expensive), so it might really be worth it if you can. Everything else is pretty trivial. Only thing you'll have to make sure is you get a CPU and MB with enough PCIe lanes for you to expand to what you want. Specifically a PCIe X4 to 6 port SATA 3 host controller. The board only uses 4x lanes, but you'll have to ensure that all 4 lanes are available or you'll see reduced read/write speeds.

Can't do a full build for that amount. Just get a MiniPC. Check out Minisforum's Refurb listings. Dirt cheap, and come with a warranty. Better hurry though...

Like a sister comment here already said, mini-ITX boards and cases often come with unexpected space contraints, and annoying things like the RAM not fitting under the CPU cooler etc.

You will probably save yourself some hassle if you go for one of these N100 NAS boards that come with a soldered on CPU and a built in cooler.

I haven’t built a PC in 10 years, I gave no idea where to start.

The only really notable change I remember being surprised by on my last build over the past decade was the shift to NVMe for desktop machines. I thought of it as laptop storage media, hadn't realized that the desktop had shifted as well.

If you're thinking about any used parts, I'd avoid buying any used Intel desktop CPUs from the 13th or 14th generations, since Intel had some serious problems, especially with the high-end models, that led to some of them suffering irreparable damage over time, and I don't know if there's any way to determine whether a CPU that someone's selling has already been damaged.

https://en.wikipedia.org/wiki/Raptor_Lake#Instability_and_degradation_issue

You should build the hardware around what your compute requirements dictate. A NAS needs little in the area if compute power but Minecraft could be a little demanding. Review the Minecraft server requirements and build based on that. Or build to Max out your budget and get the best you can up to that point.