If you're already familiar with ArduCopter then I'd suggest loading up ArduPilot into either a cheap APM 2.5 or Pixhawk - the latter if you think you'll need the extra horsepower. The last I looked ArduPilot is still in active dev/support (unlike ArduCopter). That may not be the case now, however. But it still is a strong autopilot.
"Its better than bad, its good"
Current FCs: Pixhawk, APM 2.6, Naza M V2, Naze32, Flip32+ CC3D, KK2.1.5
Aircraft: miniMax Hex, DJI 550 (clone) TBS Disco, 450 Firefly, 250 Pro, ZMR250, Hubsan X4, Bixler 2