Prelim's work load is A LOT, to prepare you for the HSC. If you want to dedicate equal amount of work to each subject and expect to do very well in all of them, it would be very difficult in my opinion.
However, if you believe that you can cope with a lot of work and if you're fairly talented academically, give it a go. You can always drop a 2u subject or drop ext.1 English if you don't like it (assuming you won't drop ext.1 maths since you need it for ext.2 obviously). If you do pursue to do Ext2 maths in year 12, definitely drop at least 1 of your subjects...
The end choice is yours but those are just my thoughts