Your recording options are limited with the PA head you have; all you can do with it is to tap into the tape outs (lower RCA jacks on front) into some recorder that can handle two signals. I don't know if it will be a stereo signal or two identical mono signals. But you could easily pull signal off the head.
I would look for dedicated units rather than dragging a PC all over the place. The recording unit could be almost anything, but these little suckers work hard for a living and are easy to carry. Might make sense getting started.
Little Fostex
Another way to go would be more complicated but would allow you to use your PA head and track up to eight signals simultaneously into eight seperate tracks that you could then mix. Start with a mic splitter for a hundred bucks:
Behringer splitter
The mics go into this unit and one direct line of each goes to the recording unit. The split signal of each goes to the PA head as you now have it. You will need more cables.
Again, a dedicated recorder will be more likely to work well in the field than a PC, which can get tempermental - not good in the middle of a set. So look at something like this - allowing you to record multiple tracks simultaneously. These units (like the little Fostex) have onboard CD burners so you can make a demo.
Bigger Fostex
You can also do some research and scour ebay for used units. Here's one like mine presently for sale. They should be available for about $ 600 - 800. You'd need to get a burner separately. I use mine to do submixes and then transfer signal to the computer, where I fool more with it.
AKAI DPS16 10 gig unit I would look for 20 G units with a v.3.0 or better operating system. I like these because the A/D converters are fantastic; they sound really good for the money. My live recording rig is a 20 gig DPS16.
Another option:
Korg D1600 MkII
It would pay to do some research, download and read manuals etc. to get a good handle on this. But there are some options available to you for under a thousand bucks. I'd stay away from flash media units as they tend to record in a compressed format like mp3 - not good enough for what you want.