It wouldn’t take much really. I run a little fanless standalone pFsense box:

• Intel® Celeron® CPU J3160 @ 1.60GHz
• Current: 1600 MHz, Max: 1601 MHz
• 4 CPUs : 1 package(s) x 4 core(s)
• 8 GB RAM
• 1 TB SSD

Last time someone asked this question, I believe the going eBay price was in the $175 to $275 USD range. Mine sits between my modem and everything else, servers, cams, PCs, laptops, et al. I haven’t experienced any bottlenecks or sluggishness.