There are so many things you can do when you make a car mod for MW that Idk where to start D:
Well I always recommend that first you just take the sample car that comes with the mod tools (in the case of MW it's that Ferrari f360) and first try to compile it and put it into the game. Once that works, make some changes to it and compile that and put that into the game. Or then just try to import a model from somewhere else.
Being able to add cars now is super cool (though I haven't tried it yet lol) but maybe for starting it might be better to replace a car instead just so you can get the mod ready faster and see how it works.
Cars have LODs in Most Wanted. They used to be super important but now with the Extra Options mod I think you can get away with having the high detail car in all the LODs as long as it's not too high poly. If you play the game with the highest detail for cars I think you only need LODs A, B and maybe C, and you can just have a low poly box for the next LODs since they aren't ever seen in the game anyway.
Note that in Most Wanted you can have several materials assigned to the same texture, even if they use different shaders. Here's a list of all the materials/shaders and also a list of texture names you can use (you can only use certain texture names like "skin2" or "engine" or "driver":http://ilmukomputerrepublik.blogspot.com/2014/01/nfs-most-wanted-basic-modding-tutorial_89.html
Here's some explanation about materials and textures:http://mwspecial.pbworks.com/w/page/8703215/ArtTextures
Here's a cool tutorial I check often, but sorry that it's in Spanish:http://retrostyle.webcindario.com/tutoriales%20-%20nfs%20most%20wanted%20parte%201.htmhttp://classicfans.webcindario.com/editando_parametros.html
In Zmodeler the scale for Most Wanted cars is 0.9843 I think. I calculated it using the Lexus bodykit 1 as reference (comparing the size in Zmodeler to the size of the real life car). You can change the scale the program uses in the settings but you may need to set it again every time you open Zmodeler.
Here are many car models ripped from games. I recommend starting with cars from NFS Carbon or Undercover maybe:http://gamemodels.ru/
If I'm not mistaken, ZModeler 2.0.8 is the one that lets you import car models from Most Wanted. It's very useful to look at those and see how they are done so you can just copy the part names and stuff for your car. Not all parts are required, you can just put the light glass and windows, etc. as part of the main car body.
I recommend getting a program called Bin Viewer too, it lets you fix the angle of the headlight "mount points" so they aren't seen "through" the car (a common bug in old mods).