Hi there! Welcome to the 405th! Glad you're excited to get going on home halo builds!
I'm not sure of a pepakura or foam unfold of Vale's helmet, but I believe a .obj file can be found in the armoury
HERE.
What sort of texture are you thinking of? And I guess another question that is relavent is what construction method are you going to be using? You asked for pepakura files, so it is my understanding that you will be doing the pepakura method or the foam method.
The reason I ask is, say you went with the foam method and your armour is entirely crafted out of EVA foam. To add some texture, you could use a sanding bit on a dremel tool, a box knife, and other tools to create physical texture and battle damage on the armour.
If you're looking to add texture by painting, here is my method: I apply a liberal wash of watered down black paint (about 50% water 50% paint) and I spread it all over the armour. Then, I take a damp rag and dab off the excess paint, leaving paint behind in crevasses and grooves where dirt would naturally collect over time. Then, I apply silver to the edges by dry brushing it on. This will create the effect of chipped paint.
Of course, there are many ways to apply texture, weathering, and battle damage to your armour. I'd recommend also looking at some of the weathering tutorials in the tutorials section of the forums (see the painting section under the detailing heading):
Tutorial Index Who can make a Tutorial? A: Anyone How do I make a tutorial? A: Create a thread in any part of the forum you think your tutorial pertains to such as Costume, Props, Soft Parts etc. How do I get the tutorial in the Tutorial Section? A: start a conversation with me...
www.405th.com
If you would like a bit more clarification on any of this please ask! I only gave you a VERY brief overview.
While it is possible to make a suit before Halloween, you may find it will actually be a tighter deadline than you may expect. Of course if you have the time to get it done then don't let me stop you! I just wanted to let you know it can take a well long time to complete a whole suit. Some members can kick out a full suit in a matter of a few months, but it is also not uncommon for suits to take over a year to complete.
Best of luck on your new build! Keep asking any questions you may have!