Sundresses can run the gamet, from super casual play dresses to work appropriate to afternoon wedding. To me, it's more about straps and material types. I'm just a big fan of buying something you can use for many occasions as well as something that isn't just like everyone else (black pants/black dress...boring!).
When I think about appropriate dresses, I'd look at Boden or Ann Taylor for ideas but you can get an appropriate one for much less if needed at Macy's, kohls or Pennies. I wouldn't buy anything from ON because I've had awful luck with their quality lately.