Baz, hi... over the years this has always been a recurring topic.. so if you are hungry for more information do searches.
Be aware that the age demographic on SB has been steadily going up (me included) so less keen to buy second and fix things using $ / time poor rationalism..
The following will likely cause a few others to comment...
Second Hand is IMO sensible for learning. Fixing a bladder while not fun is very doable. When my kids started I did not want to spend top dollar so happily fixed things and bought 100M of Dyneema and made bits up. Once you get more cashed-up / confident in your interest then spend more...
My rules of thumb.. if I am buying sight unseen I use this site and will only buy off someone who has been registered for a few years.
I review for price range and there would be no good reason for a second hand kite to be at the top of this range unless it was just about a virgin.
I would be prepared to buy from Gumtree but only with inspection = hard as a newbie..
Inspection allow for 30 plus minutes..... Always inflate a kite and then talk as you inspect.. use ^^ NKAus's guide for how much to knock the price down if you have to fix a kite (not a good idea if you aren't familiar)..
Be aware that a new bar and lines cost can easily cost $750. So when you pick up a kite have a review of the lines. Clues .. where the lines attach to the kite is a high energy are so look at their condition.. a lot of fraying - steer clear. If you can run out the lines for any length (the more the better) run them through your fingers any differences you feel - so bumps changes in the diameter indicate possible damage - have a look - fraying obvious wear - not great.
When a kite is inflated try and get to look through as much of the canopy as possible from the dark / shaded side towards the bright / sunlite side - holes become obvious. A few pin pricks no worries but lots - not good it means the kite sail becomes more porous and is more than likely fairly worn so definitely not top dollar.
An example kite, I have a 10M Lithium 2012 which I had replaced all the bladders once the main valve leaked and I had already patched bladder about 18 months back - just about to send it South to someone in Tas'- when my brother decided to have a go at kiting so I kept it. Nothing wrong with the canopy - but bound to have couple of pin pricks..you would not need to be precious with it and now worth at best $180-280 with bar and lines. At the time the bladder replacement cost $250 a bit less.. If you are prepared to spend $500 on a kite accept that you may well junk it after you have learnt.. likely you will keep parts (bar and lines).
Other equipment.. Harness target comfort over Dollars would suggest new..
Board well learning can be a scratch and dent bonanza - buy second, would target width so higher width for learning. So 40cm is a good medium point..Length hmmm 140-146 I started with a huge board but narrow board and it was silly
Assuming you have a usual learner profile - you are cautious and prefer lower winds than higher wind strength.. Essentially the bigger your board is the easier it is to get up on it. The trade off is with a bigger board it is easier to get over-powered. On balance bigger than smaller.
Get a functional pump... plus you need a safety leash - they come with new kites less with seconds...they connect your harness to total depower process (called flagging the kite) on your kite when you release the chicken loop so you don't loose everything when you detach from the chicken loop...
Turn over.. well my experience I have bought more kites than boards than harnesses... I have given away a board chucked a clunker sold and junked a kite or two and junked old harnesses. But have alot of antique kit too

Suggest always have a helmet and good idea to use an impact vest which will have some floatation.
I will happily replace lines and pigtails (the line joiner bits) at the first point of wear but will look at repair on the rest.
I think that most of the key features of the new kite are totally lost on a learner except for the price.
So until it breaks you will not notice the difference between a kite 1 to 5 years old
I am cautious and have always been in lower wind zones - someone from WA has a different outlook on good wind strength - it is what you get used to...
Kite size.. a twin tip is power hungry compared to a directional (surf board style) 85 kgs 10m and 15knots does not work well.
but by 18-20 knots with a 145 wide board fine but 30 knots too much for safety.
I favour 15-25 knots at a 12 meter kite and would be targeting a 10 m as a second kite in windier zones once I had learnt to ride the board in both directions and could hold my ground.
Should be enough food for thought!!
Cheers
AP