picksie

If you want a good one that will work, you are going to have to spend a little more. Your best bet is a ceramic one - they are the most efficient & hot but cause the least amount of damage. Also, the more expensive the less time you have to spend on one pass which amounts to less exposure to that section of hair.

I used to work with a girl who had extremely long, thick & curly hair. She used an iron and it would stay straight until she washed it again (usually two days max). I don't think that humidity will make a difference - again the quality of iron would affect that somewhat.