1-Sculptris is a bitch highpoly crap that wont be for use in games like GTA, not even any game...no game will accept you meshes of 50,000 polys that is the less you can manage in sculptris
2-Blender looks cool but Max has the scripts and everything for GTA SA modding, so try 3D Max, you dont have to buy it, just get a damn crack and ready
90% of guys in Modding dont buy it just crack it the other 10% then must be damn stupid to buy it and do free moding which wont give you money
3-ZMod is just accurate for cars, and just has lame scripts for GTA IV...so in less words i seem 3D Max the most suitable