5 watts is the maximum power you can legally run in Australia without getting a license.
There are some very powerful uhf radios avaliable like the TYT TH900 UHF that can be found on ebay which booms out 60 watts of speaker blowing illegal rf power... if you can get your head around how to program the 80 channels into it.
60 watts is great, people will hear you for miles but it won't help a bit if you can't hear them.
Hilly terrain is challenging for uhf radios but propper choice of antennas will help.
Try an antenna with 3db gain in hilly areas.
UHF antenna db gain explained hereAnother thing to watch out for is all these stubborn pricks who are still running the old 40 ch uhf radios.
The old 40 ch radios run a wide band frequency and the new 80 ch radios run a narrow band frequency, the two are not very compatible and you will have big trouble talking between them. The 40 ch will also interfere with the 80 ch.
The ACMA initially said that the 40 ch radios would become illegal but their wording has changed somewhat.
ACMA 40 CH UHF RADIO PHASED OUT read about it hereIn my opinion having a good quality 80 ch uhf is still a handy thing to have but you need to choose your antenna wisely.
Just dotnt expect it to perform over long distances. Thats where the old 27meg cb radios come in handy.