I have an Intel NUC, particularly the NUC8i5BEH I got off eBay. Unofficially supports up to 64GB of memory and has NVMe and SATA for storage. Small footprint and sips power. I currently use mine to run ESXi with a few VMs on it, sits in a cupboard and had no issues with it. Probably a bit more expensive than a small form factor PCs from the likes of Dell or Lenovo for others have suggested though.
I was watching one recently on eBay with 16 GB RAM and 1 TB SSD that went for 200 GBP. I’d go secondhand, not brand new. Anything newer than an 8th gen intel CPU, ideally quad core should be enough to get you started.