Acceptable wedding guest attire for indoor early afternoon summer wedding?

Hi! I had a wedding invitation sprung on me last minute by my boyfriend - namely, that of his best female friend. It will be held indoors, at a banquet hall (both service and reception), and the service starts at 11am. I have the brown linen dress pictured in the center of this photo (http://clothingcult.com/clothingimages/primary.jpg), and was thinking of wearing it with a narrow, leopard print calf hair belt with silver-tone buckle and grommets and brown patent pumps, and just letting my hair go natural (it's long, brown, and wavy) and going light on the makeup, and wearing an off-black silk knit cardigan during the actual service if it's cold. Think this would be okay?