Hi Waveripper,
I know you said a production kite board is out of your league $$ wise, but I reckon it is worth saving a bit longer to get something that will handle the beating. A Poly board will work, but you will ding the hell out of it with heel pressure and everytime you land on the nose or bash it with the bar, it'll take a bad hit, start leaking and taking on water, then turn brown and the foam goes soft, so it will last you a year maybe two then it will be a bin job.
If you start hitting crazy pitching lips in bigger surf, the board won't last a few weeks.
The Current Airush Converse 6' is probably exactly what you need, whether you can afford it is another thing. The bonus of the story is that you can take the straps off and surf it too. The board needs to be strong if you ride it hard.
Thruster works very well as does Quad No discernable difference in my opinion from my experience (which is limited on quads). This brings me to another option, have a look at the new Airush Choptop. It is a solid board, you'll NEVER break it, it's quad fin, 5'3" a bit like a fish, comes with full deck pad and hooks for your feet so you can still boost and air, or you can remove them for full strapless. An excellent board for junky waves and onshore slop.
For clean down the line work, the Converse, for mushy messy surf and small to medium down the line, the Choptop.
Have a look here
kiteboardingschool.com.au/index.php?option=com_content&task=view&id=21&Itemid=48Good winds,