Larry, Regardless of your analysis of "what is there" structurally, you still don't know if the structure will hold up the new loads. And I am not just talking about the dead load of the new flooring You will have preople up there, and unless you have had things looked at by a professional architect or engineer, or both, you could be looking at trouble......remember the Hyatt? Dancing motion? ETC.
There are occupancy issues, fireexiting issues ( a big one), and a few other code items that unless you have your act together, you could really find yourself on the working end of a law suite.
I assume you have been to the municipality/county agenecies and perhaps they have given you the ol' ok.......and, if you live in an area with good quality control on these issues, you could be AOK. But, if you live in "good ol' boy" land where no one, including the State and Local firemarshals are sure what is up.......take it to the pros.