Looks like I finally managed to do this script. It changes drug dealer's weapon to a random one. Pistol is not popping out anymore like in other mods that tried to do the same.
Weapons:
Unarmed;
Knife;
Bat;
Pistol;
Sawn-off shotgun;
Tec 9;
Silenced pistol.
It also changes his fighting style to a random one. Drug dealers now always have weapons in their hands. No more concealed pistols. Cons: if the area is dealer-intensive, like when there are really a lot of them — they will spawn with the same weapon. Although it's not something that happens too often.