Many of the same considerations as a freestanding fridge freezer will need to be applied when you're looking to buy a built in fridge freezer. The 2 main differences being that you need to be sure to choose one that fit's in to the kitchen unit that you currently have available and that the one you choose will need to use the same hinge system as your current appliance.
The first type of hinge is a fixed hinge. A fixed hinge means that the hinge itself if attached solidly to the unit's door and doesn't move, so when it opens, the appliance door and the unit door will open all open together in one piece.
The second type of hinge is the sliding hinge. This type of system has a rail attached to the units door which the appliance door is attached to. The unit door is attached to the units and as you open it the appliance door slides in to the hinge to open up.