Electronic – North American Regulations on 433MHz Radio

To answer your question:

"Are there Tx/Rx ICs I could use which power off until a signal is received"

No not really: receivers are "high power" devices. Even the lowest power receivers will use mAs as @pjc50 notes.

The current work on low-power radios focuses on duty-ratioing and/or time sync techniques to save power and achieve long run-times with small batteries. The goal is to minimize the time the radio is on as much as possible but still have a responsive network.

Contiki and TinyOS are two embedded operating systems with this focus. They both run on a variety of low-power wireless microcontrollers and implement different low-power control schemes for radios. You could use any of the hardware supported by these OSes to complete your project. Both have sample code that should be easy to modify for your purpose.

In free space, it doesn't matter. The power per incident area of a propagating wave is inversely proportional to the square of the distance from the transmitter. This is true regardless of frequency.

Certain frequencies are reflected, refracted, absorbed, and scattered differently by different materials. There is no single monotonic relationship until you get to really high energies, like gamma rays and beyond. At these really high energies (high frequencies), the waves basically just blast thru any material in their way, with higher energies passing thru material with less attenutation. Up to below Xray frequencies, there is no single answer, and it depends on the material between the transmitter and receiver.

Diffraction effects can make low fequencies (long wavelengths) appear to bend around objects, but this actually occurs at all wavelengths. The "near" layer where diffraction effects occur scales with wavelength, so it appears to us at a fixed human scale that long wavelengths go "around" objects where short wavelengths don't, but that is due to our perception scale. On the scale of the earth, commercial AM radio frequencies around 1 MHz are low enough to diffract around the curvature of the earth to some extent making over the horizon AM reception possible. Commercial FM radio, being 100x shorter wavelength, exhibits this effect much less for the same size earth, so FM radio appears to us to be mostly occluded by the horizon.