This is just one thing to think about...

Your UI involves multiple interactions. By that I mean the script has to do some processing in order to provide each step . E.G. In order to populate the 2nd drop-down, the script has to process the results of the 1st drop-down. Etc...

Since you're describing multiple interactions, you're going to have to either reload the page multiple times or do some ajax magic. Or you could have one page per list.

So, the slickness of your UI depends on how complex you're willing to make your code. If you want to keep it simple, you might want to make it multiple pages. (Those multiple pages could still be generated by a single script though..)