All three options are Sharepoint, either Sharepoint Server or Windows Sharepoint Services. Sahrepoint Enterprise search is overkill for searching a DNN site Sharepoint Server search spiders Sharepoint sites better than Express, but not sure if external sites are any different.
Frankly, Sharepoint Search Express is free, try it. Plan on a large database, and it may give different results from a DNN search since it doesn't searc h the DNN database, only the material it can spider. Personally, for search a DNN site better than a DNN database search tool, I'd go to a Google solution.
Jeff