Easy answer? Whatever you can afford! I am using an old blackburn mtn rack and some 20 year old Cannondale panniers that I scored off CL. The setup is perfectly serviceable and cost me under $50. Would I like a Tubus and some Ortliebs? Yes, of course. If the components in my current setup do not prove to be durable I may need to replace them, or as I save up I will replace them in due course and pass on my old stuff through CL to the next guy.

The point I guess is that while the high end stuff is nice to have, don't let it stop you from getting started today. It's not like you're riding with a full load through Mozambique or something - Some of that cheaper stuff is perfectly good for short-range light load commuter riding.

Now go shopping and have fun!