Hi, I can give you a little help. One, community mod should be loaded with patch file, not filesystem file. I only load skins and Heddly's car mods that change how stock FO2 car look.
Here what my filesytem file look like. I got 4 skin packs being loaded:
fo2a.bfs
fo2c.bfs
fo2camsfinal.bfs
car21_Crusader.bfs
MixerTruck.bfs
megaskinpack.bfs
megaskinpackfinalpart2.bfs
megaskinpackfinalpart3.bfs
aiskinpack.bfs ********************************************
You should have a patch file that list the bfs files of the community mod:
patch1.bfs
FOJs_Community_Mod.bfs
FOJs_Community_Mod_Trackpack1.bfs
FOJs_Community_Mod_Carpack1.bfs
FOJs_Carpack2_and_updates.bfs
FOJs_Community_Mod_Updates2.bfsmain_menu.bfs
start.bfsYou did list main_menu.bfs and start.bfs files in the right spot. If you going to make changes to community mod ( parts 1-5 ), bfs files need to be listed after FOJs_Community_Mod_Updates2.bfs.
Test stock community mod patch file and make sure stock community mod works:
patch1.bfs
FOJs_Community_Mod.bfs
FOJs_Community_Mod_Trackpack1.bfs
FOJs_Community_Mod_Carpack1.bfs
FOJs_Carpack2_and_updates.bfs
FOJs_Community_Mod_Updates2.bfsYou are using De Stan Flatout mod manager that come with part 5?
If stock community mod is working, then add main_menu.bfs to the patch ( drag file on to mod manager window ) and restart FO2 and see if game crashes. If it does, then the change you did is causing problems with the mod.

About loading cars, you can load carpack 1 and 2 or you can load carpack 3 and overwrite carpack 1.
Suggest you load the 2 carpacks that give you the most customs cars you want to use for LAN. And then start making a custom bfs file that will load the other custom cars you are missing. Or download some single custom car bfs files and then add to end of patch file:
patch1.bfs
FOJs_Community_Mod.bfs
FOJs_Community_Mod_Trackpack1.bfs
FOJs_Community_Mod_Carpack1.bfs
FOJs_Carpack2_and_updates.bfs
FOJs_Community_Mod_Updates2.bfs
main_menu.bfs
start.bfs
car129_Ccab.bfs
car130_41Willies.bfs
car136Viper.bfsFound this post about renumbering custom cars:
To renumber a car:
Unpack car or cars and you have a "data" folder with all the other folders inside. For example for car 46 to be car 47.Take the jupiter car46, inside "data/cars" folder you rename "car_46" to "car_47". You do the same for "data/menu/cars" , rename the menucar file and folder to 47. Now check in "data/sounds" folder , if any new engine sound files there with car 46, rename to 47(when you rename engine sounds you will need to update the carbed file in "data/cars/car number which would be 47 now as you already changed it.So scroll down to bottom of carbed file and change 46-47 for sounds as you did for carsounds files.)
You are now done accept for wheels, which you can't change without zmod. The tire will still work as your carbed file is telling the game to use tire46. Technically you can rename tire mesh.bgm to 47 then in carbed file change tire to 47 but the texture file needs to be 46 since in zmod program , tire046.dds has bin assigned to the mesh.So when tire46 called upon by game the .bgm file looks for tire046.dds automatically. If tire046.dds not found in game then it will the tire mesh will crash game saying tire046.dds not found.