I am going to lower my car and put 18"s on it. do i need to upgrade the brakes right away or can this wait??

as long as your brakes are in good working condition you won't have much problem stopping. although depending on the weight of the wheel your car may feel slower.

Just as the car feels a bit slower, stopping distances are definitely increased. I believe the impact on stopping distances will be greater than the impact on acceleration.

If you've got fully stock brakes, I definitely recommend upgrading to some better performing pads.

I recommend that to everyone... but especially if you're running bigger wheels. That way you're not taking a step backward in safety.