http://www.wwiivehicles.com/germany/half-tracks/sdKfz-251.asp
SdKfz 251 Ausf C: New single piece armor nose plate for the engine compartment. Air was drawn up behind the lower front plate. The top grille was eliminated. The side cooling flaps were replaced by side vent covers which allowed vents to be open all the time for better cooling. Changes to seating and stowage. The mudguards were raised to allow for more room between tire and guard. Tools were moved to front part of mudguards from the upper rear sides. The radio was moved under the front armor in front of the commander's seat. Used the FuG Spr Ger f radio.2
SdKfz 251 Ausf D: The armored body was simplified with a reverse sloping rear end. The air intakes were under the engine side armor and there were stowage boxes permanently fitted to the sides instead of mudguards. Used the FuG Spr Ger f radio.2
You can use both, the D is a later design.