I have used 4-5 oz. vegetan for softer messenger bags, and 8-10 oz. for heavier/rigid ones. You could also use oiled leather if you don't want to worry about dyeing, coating, and softening vegetan. I would shy away from suede because it will stain VERY easily, and upholstery leather because it's so thin and shoddy looking compared to full-grain or top-grain leathers.
A simple pattern for a leather messenger bag would be to cut a long rectangle and a short rectangle. The long one will be the back of the bag plus the top flap, while the small one will be the front. Then you just need to cut a gusset that can run three edges of the smaller bag. A shoulder strap could just be stitched to the top of both ends of the gusset. Pockets and such can be added as you like.
I hope that helps, I'm sorry I don't really have a pattern available. I usually just cut pieces out as needed, mirror them, and keep gusset blanks at various widths in my shop.
