There is a good thread here. It has a lot of examples of what people are using.
Most could probably write a small book answering the questions you ask, I know I could.
My OH server is a home grown 2ghz quad core pentium j1900 mini itx with 4gb memory and 3tb enterprise sata drive running debian. It is way overkill for OH but it was already existing for other purposes. It basically idles all day but only uses about 20watts so that was the point.
I migrated from a vera. As such, I still use the vera as my zwave controller. I have about 20 zwave lights/dimmers and outlets. I have 3 honeywell zwstat thermostats. I use the vera binding/actions to get and control zwave devices. All of my gear is 120v as I live in the US and I haven’t worked with any of my 240 devices. Although if my wife leaves the oven on one more time… arduino time.
I have a dsc alarm so I use the dsc alarm binding to include the sensors.
I have a few home grown arduino projects leveraging various sensors and relays. They communicate via mqtt.
I control/monitor room lights, irrigation, fans, doorbell, harmony hub, dht22 temp sensors, garage doors, thermostats, outlets. Mostly simple turning lights and outlets on and off at the right times.