Well, you can do some stuff just by using the editor. Thats basically change the stats and characteristics of items, people, horses. At the moment you have to go to troops.txt to change their names yourself. Its not hard to find the names.
The mod tools will be out within two weeks and maybe withing just a few days, ive heard, and that will make things a little easier but rather than learn the item/npc script youll still be better off using the editor or a new version of it compatible with those changes.
As far as changes to the quests (conversation.txt) you can do that now but its messy as we dont quite know all the codes yet. Its kind of a pain. Its unclear how much this will change, if at all, when the tools are out. Most likely Armagan will just give us the list of operations that make things happen.
New textures for the models can only be added via a mod at the moment, the hugemod. If you have a texture you want in game without using this mod, you just have to replace an existing one. Putting new textures into the mod is a pain for noo8 and might be irrelevant soon anyway.
New models arent possible at the moment.
So, those are your constraints. Id say follow the example of the two major TCs right now and wait to see how the mod tools shake out before doing any major coding. Its just a pain and probably be redundant soon anyway. They are mostly just making models and textures right now and waiting for the tools to enter them into the game.
This is the editor
http://taleworlds.com/v-web/bulletin/bb/viewtopic.php?t=1971
I think if you want to make a new item you should open your item_kinds1.txt and copy the line of an item that is the general kind you want to make. Then paste it to the bottom of the list and rename it whatever you want. Now you can use the editor to change its stats. The first entry is the name, second is the model/texture to use.
) but easy to use 
i just realized you meant models, not actual items.
