Good luck with your project!  It takes courage to post your work on a board like this.  I hope that you will post a link if you post a completed version on the web.

If you have Poser 6, there are some free toon materials in the Sixius folder under the material tab that are easy to overlook (I went months without realizing they were there!).  The colours are limited but are easy to change.  This will save you some searching and money if that is a concern.  However, using computer graphics for the art is becoming more acceptable, as the toons here on Renderosity suggest.  So don't hesitate to stick with them if you like them.

As with any type of writing, there are any number of approaches.  Comic strips and some comics (Watchmen by Alan Moore, for example) use set panel sizes and layouts, but others (manga especially) take a lot of liberties with the panel sizes and positions.  So read whatever you like with an eye to layout, pace, amount of dialogue, etc., and learn from that.  There are also some excellent books by Will Eisner and Scott McCloud that cover the basics.  I couldn't read the sample you posted (too small), but the variety of close-ups, the variation in word balloons, and the dynamic poses suggest that you're off to a good start.

Good luck!