On the Frequency, I have done both 5.8 and 1.3, I tend to lean to the lower freq. You will get longer range with the same 500mW system.
If you use a 2.4GHz radio, the 2 freq's you can use are 900MHz and 5.8GHz.
The difference between the two is, the antenna's are smaller on the 5.8 and more compact if you want to do goggles with the RX in them.
The 900mHz will go behind trees with in reason with little to no interference, unlike the 5.8gHz which tends to flicker and distort a bit when it's not LOS, line of sight when you are far out.
A 300mW 900MHz will go as far as a 500mW 5.8 GHz systems. It will be all to what suits your needs the most.