For a rear bag I use a rack bag that has integrated panniers that can be folded back up when not in use. Unfortunately the model I purchased (from REI) is no longer being produced, but there are others out there. It's kind of the best of both worlds-- panniers when you need/want them, or just a trunk bag when you don't.
Also, there's a search function on this forum at the top right of the page and you can look for whatever you're interested in. Lots of people have posted things like the accessories they use and it's all there waiting to be found!
I have a Topeak rear rack (actually two, one for non-disc and one for disc) and a Topeak MTX bag that locks/unlocks quickly on the bike. Internet pricing is around $45 for the rack, and $65 for the bag. I am selling the 2 racks and bag for $70. They were used for 3 months and are in perfect shape. PM me for more info and photos.
I like the Topeak and Ortlieb quality. Both are German vendors and can be found easily throughout North America. They have a ton of solutions.
On the lock side, I have a slightly different opinion than most. Buy the lightest and thinnest cable lock you can find. They prevent the casual walk by theft. Anyone who is a serious bike thief will cut through most anything (u-locks/chains, cables) in seconds. There are lots of videos about this. Some folks like u-locks but they do not work in every situation. Cables tend to work in more situations.
Buy a lock that is easy to carry and/or mount on the bike and is easy for you to use. The lock you like to carry is the lock you will use.