Neutral colored sand or gravel, tending toward darker rather than lighter.
Lots and lots of rocks, altho you can also make caves and stacks from PVC pipe or brand new clay pots - just not as good looking
Roof slates or other flat rocks in addition to round rocks make cave building easier.
A gravel vacuum - basically a flexible tube a couple of feet long, attached to a rigid tube a couple of feet long. It works by siphon, and NO, you don't have to suck on it to get the siphon started. This is for regular partial water changes, and the rigid tube goes into the gravel to pull the gunk out.
A couple of clean (new) buckets - to be used only for fish.
A notebook for observations and reminders.
Water pump. no. Although if you're moving a lot of water they can be a nice luxury. You might want a trash can on wheels to take water away, and a NEW hose to fill or do water changes.
For Malawi, be careful as the fish will hybridize if they are closely related species. Do research and try to get species that are not going to cross if you intend to breed. The females all look similar so it is hard to tell sometimes. A lot of people only want the males as they are more colorful. And unfortunately in the malawis a lot of times if you have one species, only the dominant male gets the color. The subdominant males look faded, more like the females.