I agree you may want to start simple, but don't get offended if your first models don't get included, just make your own mod

The hard part, however, may not be modeling, but texturing and uv mapping, so be sure to practice that.
Also, at some point you may want to look into a hipoly => lowpoly workflow, which allows for a lot of detail while preserving a low polycount (if you do it correctly, that is)
As for what to work on, most of the second and third campaign stuff has yet to be remade. However, just make anything you feel like (a new gun design maybe?) and try to get a completed model included in a mod of your own. That way, you can start your next model with the experience of which stages the model will have to go through.
Also, if you need help/feedback, just post here, or feel free to pm me
