No there is no cheat for this car in this game. Maybe on the PC but not on PS2.
That depends. If you are counting vehicles as a separate type, then 0 because there are 'Cars' and 'Trucks' but no Vehicles. If not, then 604 :D
Well, your character came back to San Andreas because he wanted to avenge the death of his mother. I don't understand the second part of your question, if you could comment on this answer and clarify I might be able to help you further.
Ride up next to a girl, and honk your horn. She'll come up to the window, and the rest is history ;)
When killing a civilian, whether it be for fun or for defense, make sure you don't kill a quest character. Killing quest characters make's it impossible to finish a quest, and many unfinished quests can bug up your game.
You don't go to the other side just like that. You gradually unlock a little more land each time. The mission when the resut of the land is unlocked is Four Leaf Clover I think.
The more you progress and beat the story, the more safehouses become unlocked. You can also get bigger safehouses too.
There might be a mod for it on the Computer, but you cannot install mods on the PS2 version of this game.
The game is similar to GTA but there are some differences. At the end, you get codes that give you unlimited ammo and such. If you want to get it, it's based on your preference.