Duel Pipes are exspensive and there may not be the space. If you really want a Duel Exhaust, Make sure it includes a balancer section Pipe, Also make sure all bends are Mandrel.
The best Silencer Brand availible in Australia is Red Back.
If you're going to keep the FZJ80 for less than 3 years...
It's highly unlikely that the Exhaust Manifold is stuffed.
Catalytic Converter. - If it does require replacing, Replace it with any reputable brand and don't be sucked into a 'Perfermance' type.
Use a Straight Through Muffler and route the tubing where it currently is. :)