In the root directory of the game you will find a folder called "Packs", in here you will find a zip file called "Pak0.zip". Open this with winrar (or any other program that reads zip files). In here you will find all the files you need to edit. Here are some edits that i found quite usefull:
Make vehicles (almost) invincible:
Inside the Pak0.zip go to the following directory:
Scripts/Cars. Here you will find an xml file for each of the games vehicle. For example the taxi (taxi.xml). Unzip the file somewhere and open it with any text editor (like notepad). Below the file you will find the lines:
<params>
<health value="350"/>
Change the health value from 350 to anything you want, something like 100000 would do nice. Now save the file, and put it back in the Pak0.zip You can do this for all the vehicles.
Cheap upgrades:
Inside the Pak0.zip go to the following directory:
Scripts/GameRules and edit the file upgrade_prices.xml For example machine gun level 1 price:
<item name="machinegun1" price="2000" />
Change it to 1 and the upgrade will cost 1.
Weapons ammo and damage:
Inside the Pak0.zip go to the following directory: Scripts/Weapons here you will find an xml file for each of the weapon of the game. To edit the rocket launcher for example open the rocket_launcher.xml. And change the following line for damage:
<damage value="800" />
Change the 800 to anything you like. Mind you there are different levels of the weapon, and each level has it's own value.
To change the clip size edit the following line:
<clipSize value="500" />