Up the punx!!!
I've played in a myrriad of different "punk" bands over the years... thrash, crust, powerviolence, screamo, pop... in all cases, the unifying 'thing' has been:
-loud
-cheap
So... for 3 years i toured with a Kustom200 head and an old ampeg 2x15... i bought both the head and cab together, used, for $400. I like it SO much, that i still break it out these days!
So... moral of the story... BUY USED!!!
The head will PROOOOOBABLY be solid state. I'm a fan of 70's and 80's built-like-a-tank solid states... Sunn, Kustom, Acoustic, and Peavey!!
The cab... i HIGHLY HIGHLY HIGHLY reccomend a 2x15. For 1, they are cheaper. I see 2x15s on the local craigslist routinely go for sub-$200. For 2, this will get you the volume you need for a loud punk band. At the same price, you may find a 4x10... but i've always felt like the 2x15's are louder (mainly due to the 2x15s being 4ohms, and the cheaper 4x10's being 8ohms). Older Peavey 2x15s are a fave of mine!
Good Luck!!!