I tour with 4 panniers, a trailer, rack top bags and handlebar bag. and a backpack for water. These are some things that I have found over the years.
Wearing a backpack can give you an ache in the shoulders and neck after a long day on the bike. It also tends to block the sweat evaporating from your back.
I have found that one pannier tends to upset the balance of the bike, especially in a head or head/cross wind.
The best set up would be to buy a set of smaller panniers designed for the front of the bike and mount them on the rear. I usually use my front panniers mounted on the rear rack for day trips. This keeps the bike balanced better and still allows me to carry stuff.
When buying panniers, make sure that you buy a pair that have lock on mounts. Nothing is worse than hitting a bump at 60 km/h and having your rear panniers launch themselves into orbit.
The other option if you only have a small amount of bits and pieces, is to buy a rack top bag for the rear rack. This sits on top of the rack and is held there by straps or velcro straps and clips.. Don't get a handlebar bag as this upsets the steering on the bike, especially in a cross wind.
As the South West of WA can get very wet at any time of the year, make sure you get waterproof bags, or bags that have covers that go over them in wet weather.
If you decide on panniers for the rear, when mounting them, make sure that they have adjustable mounts so that your heel doesn't strike them when pedalling, and that they have clearance from your gear changing mechanism.
And finally with panniers, make sure that your rack has bars to stop the panniers sliding forward when braking, and also support to stop the panniers bending inwards and striking the spokes.