Get a multimeter with a duty cycle function and read the fault codes on your car. You'll save a lot of money by not replacing perfectly good parts.
After buying a simple Craftsman meter for about $30 and getting the Service CDs, you'll be in good shape for narrowing down your problem.
I tried it the other way. Replaced a lot of parts in trying to solve my hesitation problem. Reading fault codes showed a fault with the 02 sensor.
You are correct in the the 02 sensor doesn't work until the car is warm.
You'll need a multimeter to adjust the mixture anyway.