Check out this site
. It's a regularly updated list of solid recommendations for a given budget.
Now, given that you already have a case and HDDs, the 500€ you want to spend only need to buy:
* a CPU
* a PSU
* a GPU
* a motherboard
Going off of logicalincrements, their 500€ recommendation build (With an RX580, a Ryzen R5 1600, stock cooler, an MSI B450 motherboard, 8 GB of RAM and a SeaSonic or Corsair PSU would run to about 470€ total when leaving out the parts you intend to reuse. That would give you a really solid rig.