Medium Range signal transmission {sensor output} RF Modules or Medium FM transmitter?

What about using some custom medium range FM Transmitter {300-800M} instead of RF modules
readily available in the market, the target area has some vegetation ,What would be a better strategy ?

You can not use FM band to control\monitor a system from a distance, it's legally forbidden.Instead, you should use ISM bands for that.
The vegetation shouldn't be a problem for instance at 433 MHz..