I bought my unit on banggood, my friend on the gearbest. The question is not “where” but “when”. You should wait for discounts or sales ;).
Ok, it’s easy to distinguish them. Better buy mijia (in my opinion).
It is not mandatory to sick to stock version. Currently, I’ve the latest version of mi home app on my phone and the latest firmware on the gateway. IMPORTANT: enable development options and “additional communication” before updating the gateway firmware or you will have to disassemble the device and solder things.
If you are afraid of soldering, stay on the default firmware.