We need a small machine to collect data from PLCs, power meters and sensors and push it to a server. What configuration is right?

Data collection (an IoT gateway) is usually light on CPU but heavy on interfaces and long-term stability, so do not overbuy performance. CPU and RAM: a few hundred data points at a 1-second cycle, read over Modbus or OPC UA and published via MQTT, runs fine on an Atom or Celeron class CPU with 4–8 GB RAM. If it also hosts a local database or dashboard, step up to Core i3/i5 with 8–16 GB. Ports: count the RS-485 ports, the LAN ports (separating the control network from the uplink), whether you need DI/DO, and whether 4G is needed as a backup link. Installation: DIN-rail mounting, 24 VDC input, fanless, fitted neatly in the cabinet. Data: enable store-and-forward buffering for network outages, write to an industrial SSD, and limit write frequency so the SSD does not wear early.