Though I am not familiar with the Marder series, I know that prior to Stalingrad there were no Soviet assault guns in existence.
Some PZ III were captured AFTER the battle for Stalingrad and some of these were converted into turretless assault guns with 76mm cannons, and are sometimes referred to as "SU-76i". While very few were ever converted, the idea was well recieved and served as a basis for an entirely Soviet design, the SU-76 (which got a further modernisational revision before actually being deployed, which is why it is more commonly known as SU-76m).
In short, the SU-76m did not see action until late 1943. It wasn't even around for Kursk, so it is pretty much a late war vehicle. Stalingrad really is quite a dull front when it comes to vehicles. None of the truly iconic and interesting vehicles have left the design stage at this point in the war, ie no Tigers, no IS2, no Soviet assault guns.