You are not going to be able to get much for $300.
That being said check out the SUMO PRO system.....
http://www.vocopro.com/p_sysh.html#sumopro
Normaly I do NOT recomend someoing getting a vocopro system, but with a budget of $300, this is the only thing I can think of. You will also need to purchase Karaoke (CD+G) discs for the music.