From my own personal experiece, I would go with the KK2 board, brilliantly easy to set up. But use what you have and it should be fine.
I don't use counter rotating blades on mine and it works great. I would recommend looking on youtube for the user hallstudio. He has put together some great videos on cheap easy tricopters. Also check rceplorer.se
. Lots of good instruction there. On the 750's you can easily use 10x47 sf props from gws or 10x45 sf props from gemfan. One thing to keep in mind, the longer your arms, the more stable the platform. You can probably find alot of tips on rcgroups revolving around your particular board. Invest in LOTS of zip ties. And yes, the board goes in the middle of your hub and it should have an arrow which indicates the front. Good luck in this endeavor. I have alot of fun with mine. My first tri had kk 5.5 blackboard, i then moved to a KK2 on a quadcopter, now I have a multiwii on my tricopter. The MW is somewhat of a pain to program though. Miss my other KK2. One more thing to keep in mind is what TX programming you use. Do your research. If you look, you can find what you need out there.