To start kitesurfing you will want a leading edge inflatable, water kite which is designed to take off from the water. It will be a delta or bow kite and have a smoother delivery of power than the c kites that are used for more experienced riders. The bar and lines will need to have depower capability and safety releases built into them. Ideally shorter lines will reduce the amount of power and make steering more direct so go for 20 m to begin with.
If you buy from a source where there is a comeback, should you find a fault with the kite, then there is no reason that you could not buy second hand. The only problem is that as a beginner it may be a long time before you realize that there is a problem with the kite or lines etc so your call on that one.
In total you will eventually need to have 3 kite sizes to comfortably cover all the wind conditions that you will encounter. A small kitesurfing kite for high winds, a medium size for moderate winds and a big kite for low winds. As a beginner it is most sensible to buy a good medium size kite with good wind range and stable performance and then think about buying more kites once you have some experience under your belt. You may not be able to go out in some wind conditions but as a beginner it is best to avoid high and low winds anyway.
As a rule of thumb a medium size is around 8 to 10 square meters depending on your body size and weight. If we classify male body shapes as small, medium and large you would need an 8 meter as your mid range kite if you are small and a 9 meter kite for a medium sized male etc. For females just go a size down and think of a medium kite for a small female being 7 square meters, an 8 square meter for a medium size female etc.
If in doubt about the right size, have a word with your local instructor as it is better to get it right before buying a kite. I prefer to have slightly smaller kites and fly them faster to generate the power needed but you will only discover what you like after flying your new kite for a season.