A very good question, Ilmrik. While the style of armour is ultimately going to be up to the artist drawing the concepts, standardizing the the type of armor the angels would use is rather important if each model, and therefore the army as a whole, is to appear unified. While I can't tell you what type of armor would be best, as I don't have a grounding in armor I would need to determine such a thing, I will do what I can in this post to help you out. Before I contineu, I will say it's very late, so I apologize if this post has poor Grammar... I think you'll get the idea of it all.
First off, we need to think about the angel's themselves, and their basic combat doctrine. While there may be some disagreement on it which can be expressed in this post, the way I see it is that the Angelic hosts, while committed to protecting their warriors on the battlefield, are also committed to making their units maneuverable and quick. Large suits of bulky armour would therefore be somewhat inadequete for these warriors. This delivers point one of my idea of Angelic Armour- Sleek. We don't want it clunky, we don't want it bulky- sleek and dare I say sexy is the way to go here.
Now that we have an idea of what type of armour would be best for the angels, the next step is to consider their faction. In the case of these beings, their tastes in clothing and armour needs to be as such to evoke of feeling of holiness- an armour type like the Dark Elven or Chaos Armour is out. The "bad" races of the warhammer world quite often rely on pointed edges and evil iconography in order to make their armour look imposing. While there is no set formula on how to make armour look "good", avoid ending edges in points, and try to find iconography that is somewhat neutral in appearance. Monsters with fangs and claws and other nasty bits are out- instead, runes and designs with multiple curves, gems, birds such as the phoenix, wings, and feather motifs are in. In order to add a some what holy looking element to the sketches, I have always used billowing robes- for some reason they always evoke a sense of elegance and devinity to me.
As the angels are a unified army force, and not a ragtag bunch like the orcs, their armour is going to be fairly unified looking, and not like it is just thrown together. Pretty obvious idea, but just thought i would throw that out there.
This is suppose to be a race of being that has crafting skills comparable to the Elves or the Dwarves, and as a result, the armour should have intricate detail to it- even the basic footsoldier could potentially have a suit as intricately designed as a Captain or General of the Empire.
Now that we have the basic ideas down for Angels, there's one more thing we need to remember before we proceed- they have wings. While this seems like a stupid thing to mention, it is actually important- these are what make the Angelic hosts unique. As a result, they need to protect these investments. Is there any for of armour that could be strapped to the wings that would be light but offer at least a bit more protection? Consider using leather plating here- it's the easiest way to go.
In summary, here are the points I covered:
1) Sleek and light
2) "Good" looking, not "evil"
3) Make the armour lend to the idea that the foce is unified, and not ragtag
4) Intricate, evoking a sense of superior craftsmanship.
5) Remember the wings
Now that I have set out a few guidelines to help get the juices flowing, you also need to look through the different styles of armour that have developed throughout the ages in order to help give you an idea of what style you want to pursue- all warhammer armies follow some era of armour (the Empire following the armour used in the Holy Roman Empire, for example). Type in some google searches on "Armor" and "Armour" and look through the pictures that come up. Look through books on medieval warfare and technologies at the time to see what armour was used. Lastly, use Wikipedia to see what armours you can find.
Below are a few links to help get you started. Hope they help:
http://en.wikipedia.org/wiki/Lorica_segmentatahttp://en.wikipedia.org/wiki/Chainmailhttp://en.wikipedia.org/wiki/Lamellar_armourhttp://en.wikipedia.org/wiki/Scale_armourhttp://en.wikipedia.org/wiki/Gambesonhttp://en.wikipedia.org/wiki/Cuirass (notice the elements of medieval armour at the bottom of the page)
Now for a big one:
http://en.wikipedia.org/wiki/Category:Medieval_armorThat should cover most of your bases and really give you an idea of where to start. Now, if you actually want my opinion on where to go with the armour, I will give you my suggestions...
The first is to evoke a feeling of holiness and zealous piety through the use of creating a "crusader" style. Crusaders are typically known for their great helms, chainmail hauberks and, most notable, their surcoats. The disadvantage of using this method is the armour wouldn't fit the criteria as well as other forms, and you would have to figure out how to use the fantasy setting to mold the armour to be a bit more tailored for the angelic forces.
The other option that would work well is to use a form of armour similar to eastern armour to match the exotic weaponry that has been formed for the angelic hosts. Lamellar Armour and Mongolian armour would both be great places to start for this idea, though if you must, you could use japanese armour *shudder*. Just don't make it look like Samurai armour and it's all good, I suppose.
Cheers,
RG