Your only concerns here, past personal preference as to what size speaker you want, are watts and ohms.You've all really helped me so far, thanks for that!
Are there any compatibility issues when connecting a head to a cab? Is there some way I can see if they will work together or not??
So far I think I will try to get a second hand head and cab. I'll have to give the specifics some more thought though.
An amp cannot operate properly below a certain ohmage rating. You can (in theory) pile all the ohms on an amp that you want, but you'll lose power. An amp will have it's preffered ohmage written on the back, for instance 300 watts @ 4 ohms.
Watts is a little easier: don't blow your speakers. Easiest way to do this is to buy a cabinet that can handle more watts than your amp can put out. That Avatar cab that I linked you can handle 1000w, and the GK that I linked you can only put out 300w, so you are in little to no danger of blowing out your cabinet.