[Edi]

The closest thing for items is the BaseI page of the Dom3 DB. For spells, DrPraetorious has made a spell modding guide which has a list of all spell names, numbers and effects. They are not in a searchable DB format, though, just spreadsheets.

Whether the Dom3 DB of units, nations etc will appear as a searchable DB depends on how much help I can get with it, but that prospect has been on the table a few times in the past and may be so in the future.