I use an HP microserver gen 8, which I bought second hand (300$) and upgraded with a better CPU (20$ from ebay) and extra ram (80$) and 4 2TB SSD’s (100$ per). I installed Windows server on it because I just wanted it to work in a way I’m familiar with, but a colleague of mine installed Synology OS on it. You can use the cd drive bay for the OS disk (with some tweaking). Since you already have the disks, this would fit your budget.