The type of leather really depends on a few factors.
Chrome tan is a softer material so if you want your bag to to stand up right you would need to add a stiffener to the inside. Chrome tan comes dyed and there are a lot of colors to choose from. You would have to buy multiple pieces if you want more than one color.
Veg tan comes in various animals: cow, deer, pig, sheep to name a few. Colors are usually natural, black, or various shades of browns. If you dye your own colors you can buy one piece of leather and dye it the colors you need
The amount you need will depend on the size of the bag. It's best to make a pattern so you can get a rough idea of the square footage you need. It's best to get extra for when you cut incorrectly, and you will