Ideally one that can use more than one disk so that i can expand it later when i can. Have some minimal experience with Synology since there’s one at work and i have interacted with it a couple times and like the interface, but am not married to any brand as long as it works.
Located in EU if it makes any difference.
Buy the cheapest old computer on your equivalent of Craigslist and install TrueNAS. If you want to use a lot of drives, make sure there are enough SATA connenctions on the motherboard.
You can also put an LSI SAS raid controller card flashed into IT into a PCI slot and use SAS to SATA cables. Can easily find them used on ebay for reasonable prices. And if you really grow your server, you can transition those SAS ports to point to a JBOD array with SAS ports, although that takes you from “cheap” to “cheap when compared to buying new.”
Unless you have some really high throughput needs, or are putting it in a real weak computer, I recommend software raid like zfs. No need for a real raid controller unless you just need additional sata ports. Heck, even synologys prefer software RAID these days and they have atom or arm processors.