Rynie was absolutely correct. Also, I didn't use any cheat tools to check the game. However, the reason for some of your crashes is because your starting character doesn't have the animations to wield certain weapons. In fact, I think it has animations for only a few weapons.
As for a weapons mod, it will work if it only changes existing weapons. If it adds weapons, then it won't work.
***EDIT: Wait a sec. This isn't entirely accurate. If you change only existing proto files, then the game will work. However, you may still need to edit the pro_item.msg file if the descriptions have also been altered. If you use the weapon mod's pro_item.msg file, if it has one, then your game will be messed up.