VPS in the cloud, in case you need to forward proxy ports to the Internet for a future rental. It should have at least 0.5 or 1 GB RAM and should be placed in close proximity to the rest of the hardware to make pings lower.
Proxy ports can be forwarded in 2 ways:
via the home/office Internet connection (“land” internet)
via the individual cellular WAN of each modem (“air” internet), useful when home/office Internet is not reliable.
PC
A laptop or a cheap PC or a miniPC. We can also create a residential proxy on Raspberry Pi, but I have previously encountered some power issues with it. We need the CPU power, so I prefer using a PC.
Normally Raspberry PI can serve ~ 5 proxies, a miniPC – up to 20, a PC – up to 50. It depends on how heavily the proxies are used.
Recommended but not critical: fast home “ground” (base) internet, faster than 20mbps in particular Upload.