Lol NFS HP 2 doesn't have tuning. I'd suggest Most Wanted because it has upgrading, the ability to drive through the world, police and it can be played online.
For complete freedom but lack of a good story : Carbon.
For a good deal of story and tuning possibilities : Most Wanted.
For a "legal" NFS-Feeling : Pro Street
But they are all offline (at least for PS2 I can't test it on PC)
You should get Underground [NFS:U aka NFS:U1] (the first one).
I've seen screenshots and, although is a bit old, it's awesome!
If you have NFS:U2, you should know what happened before.
I'm downloading NFS:U1 (don't ask me ho, or I'll receive an illegal violation mark).