Most of the time when I head to the junkyard it is to buy smaller parts that have broken on one of the many 15+ year old, 200K+ mile vehicles in our home fleet. For 90% of the trips, I have a very cheap, made in China set that includes 3/8" and 1/4" drive metric and SAE sockets, a two crescent wrenches, needle nose pliers, regular pliers, wire cutters, knife, and a combination screwdriver and metric and standard combination and hex head wrenches. Probably has a few other things. It came with its own fitted carrying case.
It was a $25 set ("Apollo" brand), and even though it was an el-cheapo set, it has worked well. Of course with cheap tools, you have to be somewhat gentle with them. You can't just put brute force on the sockets and such as they will break.
I also keep a can of WD40 with this set. Comes in handy...
Surprisingly, this has worked out really well. I rarely am without a tool I need. The best thing is that since it is in its own case, it is easy to carry as I move around the yard. If I know I am going to be going after some heavier parts, I will also pack a few other things. But, with the set I have I just keep it in the truck all the time.
If I were to do it again and didn't want to spend a ton of money, I would look for a set of better quality, maybe what they sell at some of the auto parts stores.