No, you cannot buy stealth cars. You can get the rough stats for all of them on there, although the F1 and SLS aren't really reachable in terms of downforce with their types, and you can't get the R10 or GT-R as light as the Stealth Models either.
The codes for the US are different. You can get different stealth cars depending on where the game and codes therefore came from but there's only one among them along with the Gamestop NASCAR.
No you wouldn't, they'd be put straight in your garage. That's how it worked with the Chrome Lines for me anyway, downloaded them after I started and it worked fine.