I want to have HA installed on my Raspberry Pi, and be able to dim my bedroom light during the night automatically. To do this, can you help me to find the proper hardware configuration? Specific questions:
1 - Should I buy z-stick or any other stick for Zigbee?
2 - Is a dimmable light enough, or I need a dimmer switch also?
3- What are the options that have worked properly with HA?
Any link to existing projects or products is appreciated 
One option. TP-Link HS220 dimmer. Assuming you have a router connects directly to wifi. I use this in my media room, works very nicely. Connects as a light control in Ha. Found automatically during configuration, simple.
Thanks, @micque for your answer. As I see this dimmer is wifi-based, can I have many of them connected to my single Raspberry Pi hub? I asked for minimal hardware but I should’ve mentioned that I want to be able to expand the system to many more devices.
I’ve got 14 switches hooked into my Raspberry Pi 3b+. Plus several outlets, security system, heating system, google chromecasts, tvs, etc. All told 50-60 different devices and my system is fine. I will say that the router is key in any setup. I’ve switched my consumer router over to open source ddwrt. Its been very stable. No performance issues.
Could you please explain more? So, your configuration does not depend on any z-wave or zigbee protocols? Only wireless? And you are saying that we need a good router with DD-WRT firmware to have a good quality connections in the network?
Yeah only wireless. There are no performance problems with any of the devices or the raspberry pi. Only problems I’ve seen in the past has been if devices fell of the network temporarily (due to distance from the router) or a power failure; how easy is it for the system to reconnect. With my consumer router originally reconnecting was hit or miss. I would see what I called network storms where disconnected devices would cause other devices to fall off until the system became inoperable. So I took the same consumer router and converted it to dd-wrt and the storms went away. With only a few devices your not going to see this but my system as I said has 50-60 devices.
Perfect. So, you have a WiFi modem and another router? Devices are connected to that router and the router to Raspberry Pi?
No just one router. I upgraded my router to open source which improved its performance. So one router satisfies all my devices, the raspberry pi, my computers, streaming to multiple chromecasts and tablets, etc.