Don't do it loosely otherwise you may end up spending a lot of time on it. If you are not a designer, tell the client that you can get a designer who can work with them first. Some clients are too anal about pixel perfect design and this client is one of those, you will spend 10s of hours just dealing with "can you tweak this button a bit to the right" type of requests.
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.
