My advice for 'special' clothing is to just have a decent windbreaker jacket and some light polypro/fleece like cloths that can be layered if the weather gets cold or coldish and damp. Buy them cheap at an outlet store, sporting goods store or whatever.

If the weather is not very hot or cold normal street cloths are fine. I find when things are more extreme having some cheap 'sport' material cloths makes the ride more fun and the day less sweaty.