Yes, and Yes,
And the reasoning is the motor and the metal in the rotor may affect GPS lock.
Are other mounting locations available and if so are they a lot more vulnerable to crashes?
I just want to get into this as cheaply as possible off the start then add functions later, I didn't wanna get the external gps because I wanna eventually upgrade to the more precise model.
http://diydrones.com/forum/topics/450-mounting-positions-lot-of-pics This will give you an Idea of the various mounting positions.
I am personally planning on using the tail block mount. With the case on the APM I assume it will be more durable in the event of a crash but I guess nothing is certain and one cannot prepare for all contingencies.
I personally have not broken a Reciever or my gyro in a heli crash and I have crashed plenty. many many times during the learning process to fly helicopters. with the way the APM is made I dont really see it being affected in a crash, Unless the rotor hits it and breaks it in pieces. But that is just my observation.
The better external GPS (uBlox6) is now available.
I have mounted mine very similarly to Wesley, on my 450. I had a couple crashes, no damage. Vibration isolation is a bit of a challenge on a 450, just due to extra bulk and weight required. The onboard GPS works just fine in this location.
On my 600, mounting is easy, and you have a lot more options. I mounted on the tray that is intended for the Rx. This obviously needs an external GPS.