You may destroy target base - can this destroy outposts before combat damage is done?
You can destroy the base first.  There is no requirement that combat points be used immediately.

From the Two Player rules link at http://www.starrealms.com/learn-to-play/

If an ability grants Combat or trade, that resource goes into a resource pool, which can be used at any point during the Main Phase.

Note that the electronic version of the game acts the same way.