Cedar is probably your most accessible option for woods resistant to rot, but it doesn't really matter as long as you pay attention to it and slap on a fresh coat of poly (or whatever you end up using) every once in a while. pine construction dimensional lumber is probably going to be the cheapest option.

Big thing is probably the feet, you don't want them resting directly on the ground or even cement if you live in a wet area. It'll make it a lot harder for water to seep into the wood and rot it. It's not really wet that damages wood but prolonged wet. Getting rained on occasionally won't do much for a long time, but sitting on dirt wicking up water day in and day out will rot the leg in a year.