A tachograph is only necessary if the vehicle is to be used for commercial purposes. If the use is entirely private then you can forget it, as you no longer have restrictions on your driving hours (or a boss to check them afterwards and a potential fine..!)
You might find that the tacho will need a sheet installed behind the speedo or it will give you odd readings – often the needle will jump right around the dial and stay there. I’ve had a few in the past like this – others have been fine without anything.
Hope this helps