Hey Kyle
Heres a handy link to the 'used buying questions' you need to ask or look at
www.seabreeze.com.au/Articles/Kitesurfing/About-Buying-Second-Hand-kites_3539609.aspxThe considerations you should be first looking at, is the wind average wind strength of where you live.. dont be afraid to post that - as you will get the locals input or even someone to buddy up with you.
At 55kg you make us all 'fat bastards'

but youll learn to drink beer n get the munchies lol.
An 18m kite in light winds means it was basically park n ride because big kites are very slow in comparison to what a 12m will do in 15 - 20 knots, producing huge power when rampaging across the back of the wind window in a dis-orientated turn or wipeout.
This is where the danger lies for a beginner... also when standing on the beach if a gust catches you out - it will lift and most likely kill you if you hit something hard... just read about the latest kite deaths around the world - kite high, big 'gusts' in average winds..
The kahoona if you can find one cheap in a 9.5 is definitely a better beginner friendly performing kite. Just because you rode an 18 by no means does it make you any better at holding a kite down in stronger winds

If you have had excelllent learning instruction & comprehension - with very good 'common sense'... a 12m waroo will be ok if the kite & bar truly are in good condition & cheap AND you get
lighter winds in your area.
I personally learnt at 80kg on a 2005 12m Naish Torch, that a friend sold me cheap as it was all I could afford back then - givin what I know now, save your pennies and be patient, youll get a decent 2012 9-10m allrounder at a good price. Post how much youve got to spend in the S.B wanted section, you'll be amazed at what you get offered sometimes (as well as dorks) - then ask your question here again telling us as much info as possible.
This will accelerate your learning curve

unlike my slow arse experience