Ahh, sorry ashfield. I misunderstood you. I thought you were trying to get your site to look like WordPress, not trying to get it to look like your current site.
It isn’t too terribly hard to convert an HTML/CSS design into a WordPress theme. Your current site design isn’t that far off from the K2 theme. I would start from there. K2’s top navigation bar is almost the same as yours, so it would make things much easier for you.
You will probably still need to tweak the header, there is good documentation on this. Here is an overview on theme development.
Basically, you are going to have to slice your design up into a header, two columns, and a footer, and put those all into different files. But you should be able to most all of your current code to pull this off. This looks like a pretty good article on how to slice it all apart.
But as joebrooksie said, depending on how dirty you want to get your hands, you might have to bring in a freelancer for some parts.