Before you go buy a bunch of wood I'd look at some composites. Some of them are pretty nice and when done, no or minimal maintenance. Wood can look nice but to keep the look you will need to paint or stain often. Most stains are recommended to be reapplied every 5 or less years. Some enjoy painting, I don't mind (but don't like) a bigger flat surface but single boards or spindles drive me bananas. Have tried various spray-painters but have never found one I like for today's thicker paints and several stains. If someone has a suggestion, feef free to add that in.
Composites are more expensive but no maintenance or curved and warping boards. Treated lumber will have some of each.