Kamie,
Let me preface this by saying that I love
Chef Mickey's. I think it is a great experience, and with kids a buffet makes for an easier dining experience.
Now to your thought that it may be too much for him. Yes, the restaurant is pretty high-energy, with a lot of ambient noise and quite a commotion from the characters and fellow diners. The open ceiling setting means that there is a lot going on visually too.
But is it too much? Possibly for a 18 month old. You know him better, and if he will react to a less than serene setting poorly. It is possible.
More importantly I would suggest that it might not be the best use of your money, in that he is unlikely to really remember any of it. He is more likely to react well to the character greeting experience within the Magic Kingdom, where there is no distraction from the focus on his interaction with Mickey.
BOTTOM LINE: You know your son best, but if he spooks easily from a busy, loud environment I would pass on Chef Mickey's on this trip. Focus on the character greeting experience within the park and save this for his next trip. If, however, he does not seem to be fazed by a lot of commotion, then keep the reservation on your agenda. But my overall inclination would be to pass on it this first time.
Write me back if you have a follow up question!
Have a magical (and delicious) vacation!