Dear Down Under:
Sounds Like you have good Sound Equipment Mackie is great stuff. I would suggest having a couple of floor/stage monitors for the singer. I personally don't use a subwoofer for Karaoke I guess if you are DJing (I'm Not) You might want to go that route.
My singers want to hear themselves on stage and when you are standing behind the Mains it is sometimes hard to hear yourself. Also having a two television or video monitors 1 for singer and 1 for audience makes it more fun for all.
You can get all kinds of deals on the internet on disks. I just bought a bunch of Disks DK brand from Karaoke.Com for $7.49 a disk.
I started with the standards Like Beatles, Beach boys, Country old and New, Pop, Frank Sinatra you get the picture and build it from there, I only have about 4000 titles but singers can always find a song to sing in my selection. I also update when singers ask for certain songs. I have also found that many people who have never sang Karaoke ask for songs that are way to hard for them the first time. Many Karaoke versions are not like the original It isn't exactly like singing to the Radio.
You don't have to be a great singer. I find if I sing the first song it is a Ice breaker and it helps to be on key even though i usually sing a full step down so I dont strain. Practice makes perfect My family thinks I'm nuts I save the singers slips and run Mock shows to practice getting rotations down and make it run more fluid. So my daughters freinds come down and join me in the fun and sing the songs. Drives her crazy that her freinds want to come over when I'm practicing my singing and shows.
Anyway If you are really going to do it and take the plunge just be prepared and also don't get offended when your family does a Karaoke intervention on you and sends you to Karaoke Anonomous Or K.A for short.
John