Your labor is valuable, don't waste your time on inferior wood that will only last a season or two at the most. Stick with exterior rated wood, like cedar, redwood or treated lumber.
You want it to look good and professional. Use the best wood you can afford. It's not like a planter box or raised garden bed.
The other Hometalkers are correct, the quality of the wood in a project is a big determiner of what you select (in your case wood) within your budget, the better the outcome and durability.