Chosen Solution

Which 2012 + airport card could be installed whit or whitout an adapter on those models to reach compatibility with handoff + continuity on Yosemite?

Sadly, Apple does not offer an upgrade Bluetooth board for your system. I don’t know of any Bluetooth 4 boards that would be compatible here. So far only combo AirPort boards (WiFi/BT) in the MacBook Pro series have been tried.

Buy a little bluetooth 4.0 dongle it doesn’t take much space and they are cheap

Would external or anything like that work ? Or any internal ones that might work ??