I am currently interested in the Cheerson CX-20 Auto-Pathfinder Quadcopter RTF aka Quanum Nova. It should be 'Open Source' version, because of the standard 'open' flight controller APM 2.5.
The device is available at Banggood for €240.95. It is also available at QuadcopterShop.nl in the Netherlands for €349.00.

Hardware wiki regarding CX-20 "Open Source", including internal images, building telemetry link, autonomous missions Ardupilot Flight Controller
APM tweaker/Waypoints etc, runs on Windows, Mac and Linux. Someone even made a Raspberry Pi version!
Youtube video regarding tuning the flight modes of the CX-20 This Gimbal from Banggood should work with APM
This Camera (Banggood) apparently works under the QC
Beginner's test
Prop Stabilizer

I found this demo showing how to connect a Gimbal to the CX-20. Most of it is not neccesary, because the auxiliary channels 2 and 3 (or channels 6 and 7) are already fed through to the bottom of the CX-20 in our version of the device (bought juli 2014). But here it goes:

You can clearly see that the receiver only has 7 channels, although the flight controller can handle 8. Channel 5 (Aux 1) is used to pass the SWA and SWB switch settings, Channel 6 corresponds to the AUX1 knob, and Channel 7 to the AUX2 knob.

Another demo connecting a gimbal, showing the correct gimbal settings. Detailed is also how to connect the gimbal power: not through the servo cable, but through the battery's change connector.

Video of constructing a DIY gimbal. Check the 'about' tab on the YouTube page for more info.

A popular and light gimbal

Translated, maybe readable, manual

Official wiki for (quad-)copters using arducopter

CX-20 Cheerson Hobbyking Quanum Nova Auto Pathfinder - RC Groups

Install misson-planner on Ubuntu 12.10 - diydrones