Yeh, Id second running it on whatever you have kicking around, and figure out where to go from there.
Refurbished SFF PCs from ebay are quite popular if you dont have anything to hand. Easy to pick up a 4 or 8 core CPU, load it with ram and pop in an SSD. Onboard NIC will be fine until it isnt. Integrated graphics for transcoding is fine until it isnt. When they start becoming a problem, figure out your next step.
Can always repurpose the old hardware as either an additional node or as a proxmox backup server.
Edit: only thing i will say: if you are virtualising opnsense (or whatever as a router), its worth getting an intel pcie 4-port network card. It will let you “bifurcate” the 4 ports, so you can do pcie-passthrough to opnsense for WAN and for LAN (home network), and virtio the nic for proxmox bridge