The reception and transmission of a 5W UHF radio is as good as the position of the antennae. Makes little difference whether it is fully installed or a hand held/
Install an outside antennae as high and as central to the vehicle roof as possible and plug it into your chosen 5W radio.
Do NOT rely on UHF for any emergency. Get yourself a PLB (Position Location Beacon) and have it registered.
Cheers,
Peter

Or a satellite phone, which you can rent for your trip.

I'm also in the market for UHF - I was thinking hand-held as cheaper option and we will probably sell our current 4wd once we get back from touring around Aussie outback. I am ttally confused about the difference except price as each brand has a range of 5w radios and I don't understand the reason to buy one over the other. Because we will be selling the vehicle I was hoping we could avoid buying and fitting aerial esp when not buying wired option.

Hoping someone could educate me so I buy the right one or at least not a dud. I may consider buying better one for use when driving if needed or if we meet other travelers along our journey and tag along with them and a cheap one so I can stay in contact with mum when I go on a short walk and leave her safely waiting for me.

GME and Uniden are both good products. GME is made in Oz.
The price differences are for options that you probably won't use. For installed units, I much prefer controls on the hand piece.
Hand helds must have their own battery. Cheaper ones may have cheaper, low capacity, short life batteries.
5W transmit power is important.
A hand held without a decent external antennae will be hit and miss. The antennae does NOT need to be a long, expensive, high gain affair. I prefer little rubber 1dB or zero gain antennae. They cost the least and are very tough. These give the clearest short range (10km) reception. The last thing I want is to hear every truckie and caravan within a 30km radius. The important ones are those who are close and you want the best reception you can get for those.
The signals (in and out) are "line of sight" and the vehicle will create a shadow effect which will greatly reduce performance behind you. A magnetic antennae on the roof and going through the door might be an option for you?
Cheers,
Peter

I have Oricom units both fixed in the 4WD and handheld. Oricom is an Australian company and I have found their products very good. They also sell the fixed units as a kit complete with matching aerial. Having both fixed and handheld is great when reversing in with the van as the missus can direct me in without the entertainment for the rest of the caravan park.