Obviously there are a huge number of variables that make each case different but does anyone have recommendations or experience they would care to share?
The point is largely to get a kick in the pants and a clearer sense of direction. So you probably want to be far enough along that you've thoroughly explored the issues and made initial attempts at development. You also tend to want a relatively cohesive team that's used to working together.
You can reapply, so "when you get accepted" may be the easiest answer. If nothing else, the application process may help you refine your ideas.
I would recommend you (the OP) go ahead and start filling out the application even if you don't submit it. I did this, in part to help me clarify my own ideas and in part to help me clarify whether or not YC was right for me. It's not but filling the application out, even only partway, was a good exercise and very helpful food for thought.