BMW: e30 325i, life span, o2 sensor

Question
It hesitates to rev up in order for me to pull away. The clutch and gearbox has been done and the engine.

Answer
Keanan,
 When was the last time a tune-up was performed on the car?  If it's hesitating, then it's probably in the ignition system.  I would change the spark plugs, spark plug wires, distributor cap and rotor, and possibly the coil if you think it needs it.  These items wear out over time, and dont last for ever.  So if it's been more than a few years I would highly suggest it.  Also when was the last time the O2 sensor was replaced, it's been said that they only have a life span of 50,000 miles, although I've put plenty more miles on them than that, with no problems.  If your engine was burning oil, or running poorly, it could have ruined the O2 sensor or clogged the catalytic converter.  
 If you could provide me with a little more info it would be great as well.  Such as, what RPM do you notice this hesitation?  Is it doing it ALL the time, or only when you put the engine under load (pulling away from a stoplight).  Those things will help determine what the problem is more than "it's hesitating".
 Hope this helps some, Good luck
 Josh