As @hvsds says Zinsser is good but it's also fairly expensive and you might only need that combination if there is staining etc @Airhead would be the best person to ask :)
For most stuff you can use the primer (not so spendy) then all coat (spendy) for a bullet proof finish. Bullseye as prep if there's particularly knotty wood involved (will need a separate knot blocker with it) or an oily surface like window putty.
On that note, what are folks go to external paints?