Another benefit of splitting up the work is that you can work in parallel. While you work on the technical bones of the application, the designer can get the theme just right. The earlier in the project that you know about complex parts that could be problematic, the better, and this is true for both theme and function. You'll still need to integrate the theme at the end, so budget at least a few days for that.
So here's what the workflow might look like:
- Developer installs modules and configures them to work together.
Designer prepares basic HTML and CSS.
- Developer fleshes out site and adds custom functionality.
Designer fleshes out individual forms/pages requiring special treatment.
- Developer ports HTML and CSS theme to Drupal.
- Developer and designer collaborate on fixing any cross-browser problems that show up.
It's usually a good idea to build the hard parts first, so you can get a better sense of how much more effort is needed and whether you need to scale things down. If you're new to Drupal, you may want to build a few easy parts first so that you can familiarize yourself with the system.
Hope that helps!