If you use the respect cheat (whatitmeanstome) and the cash cheat (cheese) until you have over 10 million go to your cell phone menu, then go to upgrades and finally go to the damage tab and buy everything (including No Ragdoll).

Once you have done this, the only way to get damaged is to get hit by a melee weapon or get hit/punched by one or more brutes.

So for the Unlimited Ammo cheat you do the same thing as the respect and cash cheat.

After that, go to the cell phone again but this time go to weapons, then fill up everything to the max or when you reach unlimited ammo. But also include the Instant Reload (it is helpful because when you reload RPGs it takes a long time).