The 10T is probably best for the 3900 based on the numbers.
Sure, stuff breaks on helicopters. It that's a problem, there's the door
. The King is ok stock, but for the motor and ESC. Other things can be improved on (of course- the gyro for example), but whether any of them are necessary is an individual decision. Of course, because you didn't buy a $100 heli and get $600 in electronics with it. But it's a good heli just the same, and you don't have to go hog-wild replacing things.