The size 18 tops is going to be an issue. Where I live several women (including myself) shop at Lane Bryant for shirts. I wear a size 18 blouse because of ahem ample cleavage. So not only is the people who are plus sized, but you'll also get people who are not, but have to fit thier chest...
I'm only 5ft but very busty, so my shirts/dresses are plus size and taken in....
http://www.casual-plus.com/html/dresses.html
The carry the same lines as a store in town, nice dresses and cheap (who wants to blow $80 on a dress they'll wear once or twice).