Hello!
After being introduced to the books in the summer of 2007, I've quickly became a big fan of the series (reading aSoS in about a week, the fastest I've read any book for fun that size). One of the things that I found really interesting was how there was so many different cultures even on Westeros. You have the Northmen, the Southrons, the reclusive people of the Vale, Ironborn and finally the Dornishmen, who in their own right consist of multiple ethnicities.
HBO seems to want to make this distinction clear, going by the use of bagpipes at the feast scene in aGoT. Now I'm not suggesting having Robb Stark wear a kilt, but here are some ideas that I've been mulling over for a few days.
Northmen - They seem to be more primitive and definitely poorer than the southern houses. I get this image that these people are stuck in an Anglo-Norman society; 11th to early 12th century Europe. When I think of these guys, I think mail hauberks, mail coifs, kite shields and lots of fur. As for helmets, I would suggest something like a half helm or a nasal helm. It's referenced a lot in the books (I think Bronn wears one during his trial by combat with the Vale knight). For this, I would highly suggest looking into the 1066 mod and asking permission to use some of the models.
Sample
Southrons - I like to imagine that Southrons are just on the brink of the Renaissance or maybe the early part of it. 13th century to early 15th century sans cannons. Lower tier soldiers might have kettle helms, swords and such.
Vale - I'm not really sure what these guys should look like. It's mountainous, so maybe look at Swiss medieval armies? Sallets and plate like the late game HRE knights?
Ironmen - Vikings basically. I have faith in you guys that you've pretty much nailed them. The only thing I would suggest is to look into the 1066 units like the Northmen for any revisions you might make.
Dornishmen - This is pretty interesting because we can actually see what Dornishmen soldiers look like. I got the comic book version of The Hedge Knight for Christmas that showed two of them getting zapped by lightning. It's been awhile, but I think they were wearing half helms too. However, we have 3 different cultures here that imo need to be represented.
-Sandy - Dark brown skin, Moorish and southern Spain, javelins, small shields and spears; armor would be something like the ghulams from Kingdoms with some heavy robes spliced on the model, heavily obscuring robes for lighter infantry
-Stony - Fair skinned with brown/blond hair, probably the guys that got shocked by lightning; northern Spain influence
-Salty - Olive skinned, Sicilian, combination of stony and sandy in terms of gear? Maybe even Byzantine influence
On a final note, in regards to Gregor and Sandor, Medieval 2: Total War Kingdoms has 2 Teutonic Knights that have helmets that look a lot like their own helmets. There's a helmet with a dog head on it (I've seen Sandor's helmet depicted from being an actual metal dog head to just having a dog head on the crest) and there's a hand on the other (Gregor's fist). Thought I might point that out as they can be used as unique general models (as could a ruby colored one for Tywin I think).
I wish you all the luck in the world. It's unfortunate I don't really have any skills as I would like to help.