A remaking of the site and a redesign are different
one just needs you to integrate a new design in place of their current, with the ability to use their current back end... this is only suggestible if their aren't usability issues etc. and with a bit of common sense should be ok to be done easily.
The remake however would involve some knowledge of how it all works, everyone has to start somewhere. It really depends on the level you want, wordpress has some neat addons it would seem that make the job very easy, while a full ecommerce CMS build wouldn't be viable...
Need to work out what is needed, what you are capable of doing to complete this, and whether that is ok or not...
Review their current site and assess the needs of the client ^_^
