well i am about to embark on a trip from brisbane to sydney in 4.5 weeks time on my old giant mountain bike....it is 6 years old and it is my old faithful.....i must admit I do treat it better than my first born (if i had one..hehe)......you don't need expensive equipment to tour. Sure some may argue that it is easier on the new lighter bikes that can be bought these days.....but you ride what ever you feel comfortable riding. I opted not to buy a new bike for this trip (much to the amusement of fellow cyclists), mainly because i am comfortable riding the bike I have....that, and I have too many memories of awesome trips i have had on it...I know it's a girlie sentimental thing...hehehe
If you are going to take a mountain bike i suggest getting some slicks put on ( town & country or Slickasaurus are ones that i have used and had no probs with). You will definately need minimum of rear panniers and a handlebar bag. handle bar bags are great for having those things that you readily need close to you...like your mobile phone, wallet,sunscreen, camera and most importantly SNACKS!! You may also want to look at changing the seat too. You need something with more padding then a road bike seat but you don't want something that is too wide otherwise you will get a nasty case of the chaif.
100km per day is a good length to start at......of course this will be just an average so that you can plan your stops etc. eg. some days might be 97km, 84km or others might be 123km.....100km per day is a nice easy average that is easy to maintain over an extended length of time. Some people find that just because they are able to ride 200-300km at a time think that they can sustain this over a couple of weeks.........unfortunately I don't know too many people who would be able to acheive this....maybe Lance Arstrong...hehe. If you know that a particular area is quite tough going ( mountain ranges, high wind area etc) you may even want to cut it back further to 50 or 60km for that day.........because no 2, 100km stretches are the same!!!!!!
Another thing i haven't asked is....how are you planning on getting back?? Riding, Driving, Flying???????? or haven't got that far in planning yet?? I know there is alot of things to think about....if i think of any more tid bits of info I will certainly let you know....otherwise feel free to ask any questions that you have and i will see if i can help you at all