If you have read Storm of Iron/Iron Warriors Omnibus, you will know that the Iron Warriors are an original founding legion and were instrumental on the Siege of Terra. As such, a number of their forces are the original (10,000 year old) marines that stood on Terra as the walls were being torn down. They later managed to steal imperial gene-seed for Abaddon and one of his crazy crusades...and are able replenish their own ranks with new CSMs. That means that it is possible for an Iron Warriors army to contain a mixture of both pre and post heresy tech.
tutorial on making great looking combi weapons *stupid*) and b) why the hell would a Champion sitting in the backfield with Missile Launchers and Autocannons need a combi-plasma??? Needless to say, this model will be swapped out for something more appropriate when the army gets larger.