Artificer's Lore and Tinker (which grants prof with artisan [tinkers] tools) from the Rock sub-race Gnome's traits seem like they should be a no brainer addition to the class. Like, take those, and the tradition offered...and I think you're mostly there. Throw in light armor...and shields, I guess. Or just get the armor via feats if you use them...and Martial Adept for more "fightery" artificer's.
Almost sounds like they might be better served as a Sorcerer Origin, but then my understanding is that they are something that can be studied/taught and learned. Not really an "innate magic" thing.
As for "Plate Warforged" then, yes they should be a sub-race if you are going to insist they should exist. Seems to me, if you want them having plate, you pay for plate like everyone else...and then just fluff it that it is "built on" instead of worn. Insisting you should be able to get a plate armored warforge PC at level 1 (by having a separate sub-race) seems like a PGing grab to me. But, I am not well-versed in Eberron's history, flavor expectations, etc...