To do the Helghan army well, you NEED to nail the helmets. Death korps, while having trench coats, at just too thin and not bulky enough to be helghast troops, and more importantly, have completely the wrong gas masks and helmets.Westwind Wehrmacht SHS
would look quite close, with some use of greenstuff to modify the face to have gasmasks. They are not prefect though.
Another area of work needs to be the weapons. The STA:52 is a bullpup assault rifle, with a linear cylinder magazine, completely different to the lasgun. Cutting down the barrel, removing the magazine and placing it behind the elbow, and adding a carryhandle sight will go some way to correcting the issue.
Drop troops isn't really the IG style, I would go with mechanised, or at least a high proportion of APCs. They do have dropships, but they don't seem to be used any more than the ISA, and they are definately not an airborne assault force like the elsyian drop troops.