Yes. You can checkout multiple items in one order, but each file will have to be estimated and added to the cart separately. You can purchase multiples of the same file by estimating it, adding it to the cart, and then increasing the quantity in the cart.

Each material has its pros and cons. In a nutshell, PLA is the best choice for most applications. It is environmentally friendly, food safe, and has great strength properties. Its only downside is it doesn't handle heat or the sun well. (PLA is temperature rated to 60 degrees - higher temperatures can cause it to deform). If you need something that will survive in a hot car during summer choose PETG, if you need something that will be outdoors in the sun or that needs to withstand temperatures of up to 100deg C, then you'll have to go with ABS. If you downloaded your file from a repository such as Thingiverse, you can check the recommended material.