The dropdown box you're describing could be bound to practically anywhere and work the way you want it to, as long as you bind it to a non-repeating field. Since it doesn't sound like the selected value needs to be persisted from one submit to the next, creating a field in a secondary data source to bind it to would be the more natural choice, but using a field in the main data source would be simpler, so perhaps you should try that first, and consider moving it out to a secondary source as a next step.
In order to have the first available value selected by default, just add an additional rule action to whatever rule set is performing the query, like this:
Action: Set [Field bound to the DDL] = [Field used for the Values in the DDL]