I always find DVDs a waste of money because I'll watch them once and never really look at them again (doesn't stop me buying em though!) but I wouldn't think that CH4 would show DH again from the very beginning, so maybe it would be your best bet to buy it. It's one of my favourite TV programmes, very funny when you get to know the characters, and surprisinlgy clever at times.
Many, many times. My memory is like a seive, so everytime I read a book it feels like the first time. I'm ashamed to say, teh books I must have read more than any other would be the Harry Potter books, I'm sure I've read the 3rd one 10+ times.