CWWK N100 - $152
1x16GB Teamgroup elite DDR5-5600 = $37
3x2TB Samsung 990 EVO - $255 1x128GB Patriot P300 -$14 (4TB storage + 2TB for redundancy and 128GB for OS)
$7 for some heatsinks for the SSDs.
Total = $465 USD
No idea what those prices look like in Europe.
You also have the option of expanding with 2 SATA ports.
You don’t need to add a motherboard, case or PSU, the first item is an entire mini PC, minus ram and storage.
The RAM and storage was just a suggestion. Change them up as you prefer.
There’s also another model if you don’t need HDD that comes with an aluminum heatsink/enclosure, for an extra $20.