How would I go about changing the characters models and adding my own ones?

Is there any good sites to get good free player models?

of course there are
you deffinately should check out openwarfare.com if you are interested in Playermodels or anything else related to cod mp modding. (they playermodels are the same as for zombies!!!)
Now if you downloaded them its pretty easy actually. you drag all the stuff from the folder you download into your raw, then find the character files for zombie_hero 0-3 .csv and .gdt in raw/character copy those to mods/yourmapname/character and replace the moddels there with the EXACT name of the xmodel of the character. After that is done you need to include the model adding the line xmodel,modelname in your mod.csv then as with a usual model put the textures form the download into your mod (mods/yourmap/images). if you get issues with xmodelsurfs or xmodelfaces not being found youll have to delete the first letter of all their names ( they are found in raw/xmodelsurf or raw/xmodelfaces obviously)
hope this helps pm me if something doesnt work
have fun with it
~HAMMY

Here are some resources where you can find some free ones:

http://www.turbosquid.com/Search/3D-Models/People
http://tf3dm.com/

To add one of the characters to COD you first need to import an existing character. Then import the new one overtop. Make sure the scale matches as close as possible. Move the rigged character to exactly match the new character(they look like one). Delete the binded mesh and then bind the joint hierarchy to the new mesh. Then you can reset transformations so the model is set to it's original position. At this point you can then paint skin weights onto the model to ensure it deforms properly. Beyond that it's just importing the model into the game.

Here's a couple videos that show the process:

Here are some resources where you can find some free ones:

http://www.turbosquid.com/Search/3D-Models/People
http://tf3dm.com/

To add one of the characters to COD you first need to import an existing character. Then import the new one overtop. Make sure the scale matches as close as possible. Move the rigged character to exactly match the new character(they look like one). Delete the binded mesh and then bind the joint hierarchy to the new mesh. Then you can reset transformations so the model is set to it's original position. At this point you can then paint skin weights onto the model to ensure it deforms properly. Beyond that it's just importing the model into the game.

Here's a couple videos that show the process:

Spoiler: click to open...

https://www.youtube.com/watch?v=L73lpDDP0gE
https://www.youtube.com/watch?v=pbMualnRVuM

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So would it be hard to get this as a character?
http://tf3dm.com/3d-model/russian-engineer-26581.html

So would it be hard to get this as a character?
http://tf3dm.com/3d-model/russian-engineer-26581.html

Poly count is a little high but other then that seems fine. To import it as should have been shown above you need some experience in a program such as maya. The tutorials should walk you through just fine though.