This would be easier for HTC to add than many think, the chassis is already in game as is the gun (SdKfz 251 and Panzer IV H).
Ammo count is not its strong point, there is not a lot of room in the SdKfz 251/22 once the gun is mounted. One printed source of mine show 38 rounds were carried, the rest were carried in support vehicles.
I'd be willing to bet that the rate of fire was fire was a wee bit higher than the Panzers.
This would be a great addition to AH, a non-perked fast moving and effective AT gun that is highly vulnerable from *any* incoming fire. Oh, and don't forget about the limited traverse. The M3/75mm would still be a superior town buster thanks to a superior HE shell and faster reload rate, but the -251/22 would be far more desirable for AT purposes with the superior AP round and FAR superior optics.