Starting to build a cheap but good quad copter

I decided to build a cheap but good quad copter.  My initial target is cost 1/2 or less of my current dji F450 with a E300 propulsion system.  And it must be just as good or better.

The 2nd quad will serve more as a FPV and UAV platform.  I ended up with the following parts:

• Reptile 550 v2 with 1 axis gimbal ($66). This is a TBS Discovery clone, which should be good for FPV and shares arms with F450, motors, esc for sharing spares. 
• Multistar 2212/920kv ($45). So I can share/use the DJI 9443 and 9450 props. I also like the easy off and on of the self locking props. I wanted a Sunnysky 2212/980kv but it seems I can't use the self locking props on it w/o the rare adapter. Another E300 set is too expensive, I can however move the E300 to it.
• Multistar 20A BLHeli ESC ($10 x 4).  Initially wanted a simonk based ESC such as the Afro.  I read there where a few issues especially for a Pixhawk flight controller.  So I opted for the BLHeli ESC.
• HK Pilot32 ($130). Always wanted a Pixhawk, I want to do some UAV and possibly coding but could not afford a 3D Robotics made one.
• Ublox Neo 7m GPS ($34)
• Minim OSD ($21). Will do more FPV, so I guess OSD is now a must.
• power distribution board ($4). Reptile V550 does not have a built in distribution board unlike a TBS Discovery.
• Nanotech apec g2 ($56)
• Walkera RX703a ($29). I have a walkera radio, as I share it flying the F450, Ladybird and Master CP.  The RX703a also use PPM signal which works for the Pilot32