Sd.Kfz. 251/9 - Schützenpanzerwagen (7.5 cm KwK37). Equipped with a 75 mm L/24 low velocity gun, using the same pedestal gun mount employed on the StuG III. Nicknamed "Stummel" ("stump"). In 1944, a revised modular gun mount was introduced to facilitate production that also incorporated a coaxial MG42. This gun mount was also used to create the Sd.Kfz. 250/8 variant.
Sd.Kfz. 251/10 - Schützenpanzerwagen (3.7 cm PaK). Equipped with a 37 mm Pak 36 anti-tank gun mount. Issued to platoon leaders as a fire support vehicle.
Sd.Kfz. 251/22 - 7.5 cm PaK40 L/46 auf Mittlerer Schützenpanzerwagen. Fitted with a 75 mm PaK 40 anti-tank gun.
Far as I know there is no 50mm or 10.5cm version, you either will probably get the low velocity 75 mm/L24 or 37mm Pak version which were numerous.
Edited 251/22 could be added, carried 22 rounds of 7.5cm ammo, it equipted the entire Pz Lehr and 2. Pz division - rather then have the 251/9 which were traded to other units.