You are not the only one to be confused over Challenge Caches, they have changed the rules so much that they are fairly useless now. However, the basic principle is that cachers have to find a set of existing caches that meet the Challenge Cache criteria in order to be allowed to find the Challenge Cache itself. From your description this is not what you are trying to do at all. In fact you are almost describing the recent Challenge concept which was quickly dropped last year.
There are other problems with your idea. You MUST specify at least one set of coordinates and a GPS MUST be needed at some stage in finding the cache. Additional logging requirements, like posting a photo before you get your smilie, are also no longer allowed. You haven’t mentioned a hidden cache at all, so what you describe is possibly closer to an Earthcache, which have another set of rules which I’ve never looked at.
I’m not much help really!