Wood will be your cheapest option but what kind of wood you use will make a difference in how long they last and hold up to the elements. Regular treated pine is the cheapest and you can stain or paint it after a few months to the color you want. You will just need to do that every few years to keep it looking good. Line inside with landscape fabric to keep the soil directly off the wood.
Plants. That is one question that will be hard to answer without knowing where the project is located. Just the city is good enough on here.
