Panniers for a Giant Defy 2.5 (2009)
Based on what I saw on the Giant Bicycle site, it looks like that model has braze-ons for attaching a standard rack - so just about any rack will work.  My personal favorite for light use (commuting and short tours) would be the Blackburn EX-1.  If you need additional support for heavier loads there is a model with an extra support strut.

For the panniers, you have a ton of choices and need to weigh factors like size, materials, attachment mechanism, water resistance.

My favorite budget panniers are the house-brand bags put out by REI in the US.
Novara Transfer Panniers

For heavier use, better performance in wet weather, etc, I would recommend you look at bags by Ortlieb and Arkel.

If you are really only carrying the bare essentials, and don't need the full packing volume you could use just one pannier, or you could use a 'rack trunk' that sits on top of the rick similar to this one made by Arkel, or the Novara Commuter Rack Trunk.  My favorite is the Novara because of the side pockets that fold out and expand into mini-panniers.

Good luck with your tour.