I'm not sure how viable getting a 40k and 30k dual purpose army is, because not only do the two armies have different units compositions that work best (30k tactical marines are apperantly pretty good, 40k ones are not), there's not a lot of units that the two eras actually share, particularly now with 40k going increasingly primaris.
Most Marine armies have everything, I think the only exclusive units are some dreadnought variants and flyers (Dark Angels don't have stormtalons, but instead have thier own flyer, etc). As for DA specially, they don't have centurions of either type, but otherwise I don't think there are any particular gaps in their unit roster. As for unique stuff, they have deathwing knights, which are....I won't call them better assault terminators, they're just different assault terminators, and they have a bunch of other terminator units like the deathwing anceint that buff other terminators. They also have elite bikers with plasma guns on the bikes and slightly better melee weapons. By the time you get some, they should have a doctrine from the newest book, which will improve the entire army significantly. But since we don't know what the doctrine is yet, it's hard to tell how the army will play because doctrines can really change up how an army works and what they can do.
As for iron warriors, they're not actually very good at armored warfare and artillery, at least not in 40k. They're the mirror image of the imperial fist, so they're decent-ish tank hunters and can deny enemies the benefit of cover, but as far as I know the tratior legions don't really have an armored warfare force the way the the loyalists do with the iron hands.