What essentials should a good wardrobe include?

[OrigamiTulip]

I'm completely lost when it comes to fashion stuff. My office is really, really laid back, so I usually wear casual black pants and a colored tshirt to work. I wear jeans and tshirts on my off days. I'm tired of it, and would like to update my wardrobe to be a little nicer, more polished, and more flexible. My wardrobe limitations are 1. I live in Florida, its hot 10 months of the year, 2. I'm plus-sized, and 3. I'm on a budget. I've stolen this list GeekyPenguin posted in a rush thread as a start, but is there anything else y'all would suggest?

- a pair or two of black or grey dress pants (the bootfit or slight flare, because they are still "trendy" but look classic compared to booty pants)
- black skirt (somewhere around kneelength)
- khaki skirt (again, somewhere around kneelength - not a mini, but can be above the knee)
- denim skirt (not a carpeter skirt, something a little more dressy - maybe an a-line with a small slit?)
- fitted tees (you can get crewneck, v-neck, split-neck, etc.) in black, white, and a few fun colors (blue, pink, whatever your favorites are)
- oxford shirts (button-up shirts with collars)
- sweater sets (cardigan and shell) in black, white, and a fun color