Im trying to get a guitar amp head kinda like a bass amp head(Rack mount) and i was wondering, I found all kinds of power amps in a sam ash magazine, but im not sure if it will work standalone since all the amps i know have both Pre and power amps...Do i need a power amp? I was thinking about using a mixer as well, but cant find anything powered under about $700.
To play through cabs you'll need a poweramp. You can run a preamp into a mixer and then through a PA as well, if thats what you really choose, but most people prefer to have the tone of a nice all tube poweramp.
You don't need a power amp, but it is highly recommended.

Look into Mesa, Marshall, and VHT - they make some great power amps under $700.
As far as I know, ENGL doesn't make any under $700.

Carvin. . . mehhh. I would rather buy an Mesa or Marshall or VHT.