#1
ok so i want a new amp my budget will be around $600 aus (could go a little higher) and i play metal like trivium and bullet and pantera but i also play some sum 41 stuff. Hit me with some suggestions. is any half stacks or anything in my price range.
#4
are you gigging? within that price range a used combo would be much more suitable to your needs, possibly a mesa f-30, jcm800, classic 30, etc..
Gear:

MIA Fender Stratocaster
Jackson Kelly KE3
Ibanez Universe
Marshall JCM 800
Orange Jim Root Terror
Mesa 2x12
Ovation Custom Balladeer
pedals and such in profile
#5
Most halfstacks under 1500 are crap
You don't need a half stack unless you're playing a stadium
A combo of equal wattage or less wattage will be applicable.
For your budget

B-52 AT 112 combo
Randall RG50TC combo
Bugera 6260 combo
Used Peavey 5150 combo
Used Peavey Ultra combo
Used Peavey XXX combo

Any of those will suit you fine