ganeshXL’s advice is perfect. Having the xHTML/CSS ready all you need to do is tweak it. You will also need to add some snippets to get the search and some other things going. The good news is that it can all be done with ’standard’ snippets (wayfinder, ditto, ajaxsearch) so it should go smoothly.
I recommend that you give it a go and let us know if you get into any trouble (this is a good project to get your hands dirty with MODx), I am sure someone will help bail you out if needed.
