Hello everybody ,
I am thinking of adding a 914 transmission in a beetle , I will be draining the oil to check what is inside , what else would I want to check ? Are they generally durable transmissions ? Do they often fail ? What is more common when buying a used one ?

You should post this in the garage. First in a beetle, you will need to flip the ring an pinion or you will have 5 gears in reverse. You would be better off finding a 911 901 gear box, which should be easier to adapt to the Beetle. Otherwise the 914 gearbox is very strong, except for first gear which is cantilevered off of the main shaft with reverse. So baby 1st gear and abuse 2nd - 5th. I would not do burn outs in 1st with a 2180 motor, it will grenade eventually...

you need to buy a 911 gear box

the cost of flipping the R/P of a 914 one would probably exceed the cost