different custom p looks in one post

Hi
I am developing a theme in which the writer of a post should be able to add classes to certain paragraphs of the content. The result being differently styled paragraphs caused bij the CSS hooking to the classes.
I am thinking of extra buttons in the backend. TinyMCE like.
-Is there such a plugin and if so which one?
-Is such a thing possible? I read WP removes classes when saving a post
-If not what would be a good solution?

The visual result must be a few custom looking banners or ads in a sidebar with editable content from the original post. (I don’t want to use intricate solutions with extra fields)

thx