Andrew,
it’s fairly easy to create a template from an existing design.
IF I can do it, anyone can.
If you want to contract someone to do it, then fine, but if you really want to learn and understand Modx then it’s probably a good place to start.
A design like yours which is tableless and fairly clean should be easy.
You should be able to find all the info either in the modx documentation or the forums.
But basically you want to do a few things.
Create a new template (or modify one of the deault templates) and copy and paste all of your header code in there.
Create a chunk called footer and paste your footer code in there.
You should be able to use your existing stylesheet without too much modification, if any. You just need to put them in the right folder - typically assets/templates/your new template/css etc.
The only thing you might struggle with at first is wayfinder and getting the menus to display as you want them. But again, Wayfinder is one of the basics of Modx.
As I said learning this stuff will help your understanding and confidence with Modx enormously.
Even if you get stuck just post a question in the forums. You’ll be surprised how helpful and generous people are.
Hope this helps.
As I said, these