1: LV imitated Marilyn Monroe, Shirley Bassey, Gracie Fields, Marlene Deitrich, and Judy Garland. There may be others, but I forget. If you have the DVD and put on the closed captions during her singing/imitations, it tells you who she is imitating.
2: I guess you would call these songs oldies. I'm really not sure.
3: Mama Cass has one of the most famous versions of "Dream A Little Dream" that I know of. You should get it, it's very wonderful. However, LV never sings it in the film, to my knowledge.
4: Limewire is a very good downloading site. Safe, complete, and free. Try www.limewire.com and you should be all set. You should be able to find tons and tons of songs by these artists, and you can probably find LV's versions. If not, you can buy the soundtrack at www.amazon.com.
I hope that helps you!! Good luck!
Little Voice is such a great movie and Brenda Blethyn is a damn talented actress!!!
There's a CD called "The further adventures of Little Voice", in which Jane Horrocks sings a collection of songs in the voices of the artists she uses in the film (Judy Garland, Marilyn Monroe, etc). "Dream a little dream" is among them - I believe she uses Marilyn's voice for that one. There's also a duet with Ewan McGregor on it. :) I greatly recommend it - I liked it better than the original soundtrack for the movie!