Have designed a Template, now what?

Hi,

I’ve been a web developer here in Scotland for about 1+half years professionally. Well, professionally for about half that! ??

The most recent project I’ve to work on is for a blog-style ‘2.0’ site that a client wishes to add images to a gallery, post articles/blogs, edit pages etc.
Visitors can post comments on each blog article etc., and full of all the usual web 2.0 jazz.

Now, I have my own CMS that I custom-built couple of years ago, with PHP and MySQL which is pretty robust, but is missing a lot of the features(obviously!) that WordPress can offer.

Now I wonder, can I use WordPress on this commercial project, or it illegal?

And a follow-up, is it easy to ‘customise’ WordPress to suit my own template. I’m guessing it will be.

Like I said, most of the work in this project is the CSS/XHTML template, Flash header and Accessibility and I’d just be plugging the template in.

Many thanks for reading this and any insight you can bring.