Hi Christian,
Yes the timeframe should place Pak 40's at Stalingrad, although they would not be as popular at this time as they would be in later years with the Pak 38 still being a major part of AT gun forces.
The gun Jerry mentions is most likely a Pak 97/38, which consisted of captured French Cannon de 1897 WW1 artillery gun that being of a larger calibre than the 50mm Pak 38 were used as a stop gap AT gun against the heavily armoured T-34's and KV-1's, and were mounted on a Pak 38 carrige. (Dragon and Italeri do make kits of these if you are interested). However, these were never a popular weapon due to low muzzle velocity and vicious recoil and were re-located to 2nd line units as soon as Pak 40's became available in large numbers.
As for the Korsun Pocket Gen 2 kit, while it is a nice kit, It depicts later war soldiers than the stalingrad timeframe. The uniforms as Jerry mentions would not really have been seen in Stalingrad, and some of the weapons in the kit are also a bit late for your purposes. The MG-42 for example would only just be seeing action and the Gweher '43 rifle was yet to be seen at all, however, I'm sure with a bit of alteration you may be able to pull it off without any major issues.
If you are interested, I built a Korsun pocket dio a little while ago using both the kits you mention, which may be of some use to you for ideas. The gallery can be seen
HERE.
HTH,
James
