I am a "mommy-come-lately" and have a 23 year-old married son, a 20 year-old college daughter, and a 7 year old son...all with the same father and all "planned" (relatively speaking). While some of the dynamics change when you have an age gap, some of the things remain the same. I was particularly concerned that my oldest would be heading off to college within a few years of our sons birth and that maybe they wouldn't bond well as a result. That has been completely unfounded at this point. Our youngest thinks his older brother is the coolest guy walking and the oldest, while not around home as much, has found ways to engage his little brother through doing tricks and stunts with him (things I could never do), playing co-operative games with him, and had his little brother be ring bearer in his wedding.
Our daughter has been home with our youngest a few more years, but she too helps him develop his karate skills (she is a black belt), plays video games with him at times, shares his interests in
Lego and Disney, etc. While she was still home, she was always more than generous to act as a "free" babysitter so my husband and I could go out occasionally. (That is one of the major advantages to having an older child at home and one which we tried to be very careful not to abuse. She was generous to offer, as her gift to us, for special occasions and sometimes it just meant that we put the youngest to bed while she remained home as the responsible party.) My husband and I were well aware that "we" were the ones who chose to have another child and that our newest family member was not our daughter's responsibility at all.
I had originally thought that with such a large age-gap, we would be free of all of the sibling bickering, etc., but we still occasionally hear shouts of, "I told you to stay out of my room!" and "Don't touch me!" and "Stay on your own side of the car!" Of course all of this stems from a little brother who just adores his older siblings so much that he doesn't realize that they need their space too.
Something that was unique in MY personal experience, which may not apply to you at all, was that when I had our youngest, I no longer fit in as well with the group of friends I had connected with originally who had older kids in high school/heading off to college, because I was now "tied down" with a baby all over again while they were looking at getting jobs, free to go out for long lunches, traveling, etc. but I also didn't fit in AT ALL with other young moms who were just starting their younger families. Just something different to consider with all of the rest of the decisions you are making.
Families come in all shapes and sizes and if you and your husband feel confident that adding another child is right for your situation, then I say, "Go for it!" Your kids will amaze you with their adaptability and capacity to care for one another. Starting over is definitely a decision to not take without careful consideration though. You need to be aware of both what you are gaining and what you may be giving up in terms of saving for the additional college costs, freedom in later life, ability to travel, etc. I wouldn't change anything about our situation, but based on some of the discussion above, I can definitely see where it maybe wasn't the most optimal choice for all families. Only you and your husband can determine what is right for your family.