Im a Uni student studying a bachelor of photography and i have quite a keen interest in photography, and im pretty sure i want to pursue it as a profession. Im not yet 100% sure what area i want to specialise in, but would love to do plenty of sports photography (cricket) and possibly some wedding and portraiture photography on the side. Im currently looking very, very hard at buying a 70-200mm lens, but cant decide on which one to get. As I am a uni student, Im purely looking a the 2nd hand market and have a budget (that i could push a little) of around $1500-1800 maxish. My dilemma is: Will the f4 IS be a fast enough lens for the photographic areas im looking at, or is it inevitable that ill need the 2.8 IS? If the latter is the case, Is the mark I 2.8 still a worthwhile lens, or will i forever regret not saving up the extra money to get the mark II?

Im sure this is a fairly hot debate, and obviously the mark II is the best all round lens at the moment, but the f4 IS is still renowned for its excellent optics, I just dont yet have enough experience to know if its fast enough for low light indoors, or night time sports.