Hi Kelso, you shouldn't need to get cedar. At our Lowes, the regular 2x6, 2x8, etc. is not treated. This is what I use, and have had no problems. However, exterior plywood with 2x4 and 4x4 dimensional framing will work well too. 5/8 or 3/4 inch is the ticket here. Keep the "nice" side to the outside for finishing. If you put a barrier between the wood and the soil (recommended) such as plastic, you'll have it made. This type of approach is standard practice up here and is effective. There are other ways of doing what you want as well that do not include wood touching soil such as permatuffa, ferro-cement, and cement permeated canvas. All are strong, attractive, and easy to build. For those who object to standard iron-based concrete, magnesium based cements are the ticket.Magnesium-based cement has about 70-90% less embodied energy than standard iron-based cement. This structure can be lighter than wood, and inert to normal chemical attacks such as acid, and high alkaline situations. If you would like more info on these alternative construction techniques, let me know.
Regards from Houston, Alaska
-Tom and Vania