Should I buy a retired Arduino? I am a beginner who would like to buy an Arduino Yún (58 dollars). 

It is retired so will I have any problem?

Else I can buy an Arduino Uno and an Arduino Yún Shield. But in this case I don't understand the difference between an Arduino Uno + Arduino Yún Shield and an Arduino Uno + ESP8266 serial WIFI wireless module.

Arduino Yún Shield: I can't find the price. I only found an Iduino.


ESP8266 serial WIFI wireless module: 7 dollars.
It is retired so will I have any problem?

It depends on what you want to do with your arduino. If you need a project that requires future support then dont buy a retired board. If you just want to do basic beginners stuff (like blinking LEDs, controlling motors, sending serial information wired or with an add on shield to send wireless information)-then any arduino can do these basic projects and probably at a cheaper price than 58 dollars.