In addition to the processor, the other important purchase you should focus on is the motherboard because it will make or break the performance of all the components that will be attached later.
The type of motherboard you choose will depend on the CPU chipset you choose.
If you plan to have several HDDs, for example, for a media server, be sure to choose a motherboard that has 6 SATA ports.
Usually, mini-ITX and micro-ATX motherboards include 4 or 6 SATA ports.
For example, when I bought an ASUS AM1I-A mini-ITX motherboard, I realized that it had only two SATA ports, so I was only able to use one hard disk in addition to the optical drive.
If you plan to use Wireless connection, it is good to choose a motherboard with built-in WiFi and Bluetooth capability.