I agree with Shackman Donna. You are building a quality unit. Why spoil it with something 80-90% of us have trouble with not working correctly or not working at all. Someone suggested giving a coupon with each unit for a digital thermometer/probe. For what it's worth, I believe that's the way to go. This is all assuming that the "probe" is for checking internal temp of meat. If you are talking about the temp inside the unit itself, well that may be a whole nother story.
Have a great weekend!