I would be looking for a pair, a designer and a developer, that have worked together in the past. If they have successfully worked together on a project before then you will avoid the largest complications you are likely to encounter; The programmer that doesn't understand and respect the design, and the designer that doesn't understand what is possible (or possible but insanely difficult)
My experience is that you do not want automatons. You want two people who feel comfortable with each other and respect each role enough to be able to have a give and take.
Something you never want to hear from your developer is (and I've heard this before)
"I did it that way because that is how it was designed. I would have only taken 2 hours instead of 4 days if I could have just..."
I wish he had come to before he had done the work. We would have sat with the designer and would have changed the design.
On the other side, designers spend more time thinking about design, and sometimes the "crazy" stuff that they come up with can really challenge and stretch a developer to push beyond what they already know is possible and create new solutions.
Ask your designer what developers he has worked with in the past. If a web designer cannot recommend a developer that he's worked with in the past, get a different designer.