Hi newly wed, I dJ and would advise against karaoke lol.
Too many drunks using your equipment, too many bad singers, the ballache of keeping your song books updated etc etc plus the big thing is you will get less money for karaoke than you will for being an ordinary DJ. I very rarely get asked if I do karaoke thankfully.
As far as the laptop goes, 95% of the stuff I play is straight off the laptop, but in effect its like having a CD player, you still need to wire it through a mixing desk, amplifier, speakers but it is a god send.
Let me know how he gets on!