What range or variation should we expect from gyroscope in a helicopter?
3-DoF magnetic gyroscopes are a dime a dozen nowadays, together with 3-DoF accelerometers, magnetic field sensors, and GPS position transducers. These are in mobile phones now, tiny and frugal with electricity. A google search for "9-dof position acceleration gyroscope" results in a lot of options.

For the sampling rate: this needs to be at least twice the frequency of the fastest vibration - more is better, since a frequency at half the sampling rate shows up as a triangular wave.



For the frequency range of angular displacement, this article might be of interest. It shows vibration data of a car driving over the road, using an iPhone to measure the vibrations. The article mentions a sprung frequency (inside the vehicle) of 1 - 5 Hz. A helicopter usually has higher inertia than a car, so sprung frequency should be lower. If you take 0 - 10 Hz as a baseline for the range, you should be safe. The ground reaction angular frequency range is always higher than the airborne one: responses in the hover are measured in seconds, not in Hz.



Above graph shows pitch response of an example helicopter, from Prouty, Helicopter Performance, Stability and Control. A helicopter with a rotor mounted on top is unstable in the hover, but in order to still be controllable for human pilots the response frequency may not be too high - it turns out that humans can comfortably control instabilities with a time period in the order of around 10 sec. The military have requirements for the stability response in MIL-H-8501A