Hi, i live in the NT and I have had no problem with the humidity with green beans - and last season was our biggest wet on record.
I do live in an aircon house though, I dont think they would go so well stored in the garden shed as an example!!
Start off with something that roasts well as a SO, like the Peru Ceja de Selva Estate, see how 2.5kg of that holds up over the time it takes you to use it, then if its ok buy 2 varieties for 5KG next time. I reckon you will be fine.