Having a child with special needs is a very real possibility if you have another pregnancy and it is therefore wise for you to seriously consider "how" you and your family would deal with it. I think in your case, with your history, this is more of a possibility than the "luck of the draw".
Very few parents of special needs children choose this lifestyle, other than those that adopt special needs children. It was certainly not something I anticipated, but the reality is PE gave me a child with special needs.
As I recall, you were understandably quite freaked out when Cate had to have a bronch for possible tracheamalacia. Many preterm babies, like my son have airway issues resulting in a tracheostomy, which has involved numerous bronchs and surgeries, not to mention the constant risk of respiratory distress. With what you have already been through with 2 children with medical issues, having a child with special needs is 100x the stress level and 100x more, if the child's issues are permanent, all of which is wise to consider in your decision making.