Roll cages need to be engineered if its a full cage..... u'll get away with a have cage, though u obviously cant have rear seats or rear seat belts.
As for interior, theres nothing in vic roads (according to my rwc place) thats says you need carpet. But i rekon u'll need some plastic coverings as there a sharp things behind them.
The best (cheap) way to loose weight from taking interior stuff out is sripping the sound deading from the chassis.... we weighed all of mine when we pulle dit out...total of 14kgs.