I agree with the cotton or linen - I made a medieval style dress for my senior prom and I used a medium weight cotton with a sheer sleeve. I would avoid anything shiney because with that much fabric, it may end up being too much. I'm not very technical when it comes to historical clothing - I make it quite a bit for faires and theatre, but I think natural fabrics would be the way to go.
Here's the pattern I used for my prom dress...
I made thw brown one and it pretty much looked the same as the picture.
One common mistake that people make when choosing fabric for these types of costumes is using stretch velvet. I abhor the stuff, it doesn't drape well, it doesn't breath, and it looks awful!
Again, something in a medium weight natural fabric will breath, drape well, and look good!
Another thought, make sure it is heavy enough because that dress will need structure and a fabric that is too light will not look right. Also, while an upholstery weight would be too heavy (though not all are the same weight, so don't avoid that aisle) something that feels heavy when you are buying it won't feel so heavy when you are wearing it. The dress is designed to distribute the weight of the fabric appropriately. For example, my wedding dress probably weighed a good 15 pounds but I wore it all night without realizing it!