You're right, for this project you'll need to edit or remake a clothing mesh and make the textures for it, and maybe make a new hair or accessory mesh for the head piece (not sure what that means here) - the face should be done in Bodyshop on a new sim.
Now it's not an easy job, and you'll need to go through several tutorials to learn the basics. When you say that you made a new model, is that an object that works in your game? If so, at least you have some notions of the SimPE interface and it will help.
The best place to start is the Bodyshop meshing tutorials in the create section at MTS2, look for the Unimesh tutorials and try all 3 of them; every step of the process is explained clearly, and there are a lot of tips and useful informations.
When this is done and understood, you can attempt hair or accessories - these are more advanced stuff, with particulat quirks; there also are tutorials for that, but they're not intended for beginners.
All this can be very daunting at first, but with a little practice it gets a lot less scary - just be aware that it will take much more than a few hours or a few days
