So tell the client that you are happy to code the WordPress site but how specific of a design are they looking for ? If pixel perfect, you need a dedicated designer. Otherwise, you may be able to get the client to work with existing templates in WordPress BUT those are also dangerous since it leaves too much room for interpretation.
I would start by asking the client "Are you looking for a specific design that is done exactly to your specs or are you ok with using existing templates" ? Their answer may give you a clue. Also, remember that design itself could be lot more work than coding the actual WP site.