Your choice of quadcopter is not bad, pretty simple. With Ardupilot there are allot more options with what you can do. You can also use the Ardupilot for planes if you ever wanted to expand. I think if you ever what to expand you should choose Ardupilot. But if your just want the quadcopter and not have any sort of problems setting everything up go ahead. I would highly recommend the Ardupilot, its a great piece of expandable technology.
Learning the APM is really easy. They've made the interface really easy, just pick and click really. The APM will take a little time though because you have to build most of the quadcopter. You could start out with the pre built quadcopter if you want fun now but if your thinking about the future go with APM. The APM will get a little expensive at first but for the capabilities and the various uses its well worth it. Do you have a budget if you were to build your own quadcopter? Will you get any accessories with the APM? GPS, voltage sensor, magnometer, ultra sonic range finder?
Go ahead and try it out on you 450 heli. For the range finder, from what i know it sends out ultrasonic sounds which bounce off objects like a bat. I think it has a range of something around ten feet and it helps out with stabilization. I don't think it is necessary though for just starting out. Let me see if i can find something a little more helpful.
Keep me updated with you progress.
I found a post about the ultrasonic sensor. Ask around in that post if you want to use the sensor. Also I forgot to mention that for the 450 heli you can use the traditional rotor hardware but I heard that flybarless rotor head is better for control. In another thread I found I believe they talk about a flybarless head but for a larger heli but i think you would be able to find the info relavent, ask around to double check and get personal opinions.
Flybarless Head (http://diydrones.com/forum/topics/600-size-heli?id=705844%3ATopic%3...)
Airspeed sensor is not needed until you start working with planes. As for the xbee its not like you wont be able to fly but they are very very helpful. With out the xbees you cannot lode code, data points, or recieve any flight info to or from the APM while the heli is operating. To lode anything you would have to plug in the USB cable every time. If you don't buy the xbees now they are always an open option for later. I was looking at flybarless heads and where are you thinking of getting yours?
whew.. i just read the APM helicopter thread..............dont know if i want to take that route haha
Just take your time and be patient.