the best wood material for garden furniture by far and away is teak - in the quantities you are talking about though it will be horrifically expensive and not exactly eco-friendly.
a good second would be oak, but again its a very expensive material and neither teak nor oak are easy materials to work with if you are not au faix as they are both hardwoods - takes its toll on the ole chisels let me tell you.
cheaper and easier would be pressure treated softwoods such as pine.
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