Living in the heart of tornado alley, having a good shelter is very important. My thoughts are if you are going to go ahead and build on for tornados, it could be also built as a bomb / fall out shelter too with not too much more.
evab, I'm a Storm Chaser in your area (Tulsa - KTUL) and I have contacts with a good company in Catoosa that builds shelters and can "customize" them for you. I don't work for them and only mention them because we are working on a shelter for our home that we will purchase next year.
If you think you've got any questions, send me a PM and I'll try and help you find out what you would like to know.
Welcome to the boards, BTW.