the number of children you want to have (or can agree on having with your significant other) has been a big topic in our household. but whatever you decide, it has to be a decision that both you and your fiance come to together.
while i feel that after having #1 and #2 a year apart, the 3rd one won't be as hectic. sounds like you have a lot to consider. talk with your fiance and find out the reasons that are important to him and let him know the reasons that are important to you.
you also mentioned medical issues regarding your health and health insurance. pregnancy is a strain on your body. if you already have existing problems, it can put even more strain on your body. in addition, the more medical issues you have, the more likely you are to have a high risk pregnancy or have complications with pregnancy or delivery. hopefully none of those things will happen, but if they do, it will put more stress on your family.
i'm currently pregnancy with #4. this pregnancy by far has been the most difficult. this pregnancy alone has been more difficult that the other 3 put together. it started out with extreme nausea, fatigue, and motion sensitivity in the 1st trimester that i had to spend a lot of days in bed (thank goodness not bad enough to require hospitalization). my girls were really understanding for being so young (7,4, and 3). they are also at an age that they are more independent--dress themselves, get some of their own snacks, etc. but a lot of responsibilities were put onto my husband.
of course he was understanding and willing to help out more. after all i was carrying his child. then things got better until about 25 weeks when i was put on bed rest.
although i don't think there is really a time when you and your fiance will feel ready. but after discussion of all the pros and cons, you'll feel like you have a better understanding of each other.
good luck with whatever you decide.