Hi Deren,
When I did mine I replaced tensioner and idler bearings, water pump and the timing belt at the same time. I also replaced the Idler bolt and the tensioner stud as they sometimes break.
I got the bearings from Bearing Wholesalers in Bayswater, the pump from Bursons (it is the same as VL commodore - some say that there is a difference but only the RB20 is different from the VL commodore) and the belt from Nissan in Oakley. Shop around a few Nissan dealers for the belt as I was quoted up to $120 but eventually got it for $80.
Total cost for the whole job was about $350.
Cheers.