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Well, you could use WF-Sections for the actual jokes. That would allow you to have categories and subcategories as well as allowing your visitors to rate the jokes and I am fairly certain that there is a good search function in that module.
For advertising space, look at the banners module and possibly partners as well.
Shopping - there we have some third party modules... can't remember the names now... something with k-shop should be quite good, but I have not experience with that kind of modules.
I'm sure that if there is someone with a desire for a "showcase" or want to be part of the site, then he or she would love to do a theme for you - but I don't really. So for me, it is a matter of doing some work and being paid to do so
Making a theme sounds like a very simple thing, but it can easily take 3-4 hours or more if there are special desires from the customer or if the themer does a little, then asks for feedback, then implements that and asks for more feedback from the customer etc.
I hope you find one to do it for you, though.
