What I can say, J., having done what you are looking to do, is that it was one of the best experiences of my life. And yes, you are right about the financial security, especially the part about being able to finance college for your children.
Our parents weren't nearly as willing in their day to live overseas, J.. Our generation is. We have a global economy now. Your father is thinking old school. I hope that you will remember that this is a small amount of time in your life to do this. Your husband's willingness to take a job overseas for his company will distinguish him amongst his coworkers, above those who are not willing to. He is "added value" to his company, and this is well worth spending 5 years abroad for the sake of his ongoing career. You are very lucky to be asked to go to a country where the "misery index" is low. You could be looking at some places in the Middle East, or in India, places that are very hard.
Another nice thing about living overseas is that in some ways, you can have your cake AND eat it too, J.. You can have experiences AND material possessions. When I say material possessions, I am not talking about a big house with expensive furniture and keeping up with the Joneses. In fact, most expats overseas don't have a tremendous amount of "stuff", because it's a pain in the rear to deal with and the company usually doesn't move more than can fit in a 40 foot container. (Some companies, 20...) The "eating it too" is more about not having to do "without", in any way. The "having the cake" at the same time as not having to do without, is being able to be part of a different culture, revel in the differences, be surprised about the similarities, and find out that you are not the same small town gal that you were before stepping out into this big wide world. And just the act of doing this with your children can make their lives richer than you could EVER afford to do otherwise.
My son wrote a college essay about an overseas experience he had when he was 13 years old. He is a very talented and smart young man, BUT, I am pretty sure that what he was able to get across about himself and how his overseas experiences expanded his horizons, is a good part of what got him a very handsome scholarship.
I do think the fact that you are planning on continuing to homeschool will give you and your children even MORE opportunities to broaden their horizons. The places you make plans to go will be part of your curriculum to study - indeed, your children can help you plan your trip to Beijing and The Great Wall of China, The Summer Palace, Tiananmen Square, The Forbidden City, and a short flight away to Xi'an to see the terracota warriors. A trip to Cambodia's Angkor Wat would amaze you AND your kids. The learning that they do to prepare for these trips would be more than they could ever learn stateside.
I hope you'll take a leap of faith and embrace the idea. Don't worry about your dad's reservations - he will not understand until he sees your happiness. A parent is supposed to want his children to be happy, J.. Don't let him keep your family from finding a happiness that is different from his. It is okay to have a different kind of happiness.