question about NFC and Bluetooth antenna


I'm really new to this filed and don't know anything about antenna, please forget me if the question is stupid. 

I have an nRF52 DK, I noticed the Bluetooth antenna on it is PCB antenna,  so my question is 

1) if I have to design my own, will it be hard or I can do the same on the development board?

2) or keep the same circuit design in the development board, but add an attached antenna, what specifications should I follow to pick the antenna?

And I know the NFC antenna on nRF 52 DK is not commercialized, so how do I find a replacement? 

(we want to use NFC to do some cellphone authentication, like between a cellphone and nRF52, so the distance will be close, like 3-5cm, but inside a plastic enclosure)

Any suggestion will be helpful.