Hi LadyBug,

Pressure of the pump is important and how high the pump has to push the water (in feet). $20 dollars doesn't sound as the pump that could push water very high. Buy the pump and see if it can push the water up the wall (hopefully it can push three feet). Then put a header line along the top of the wall and just drill holes along the header line (less maintenance) or insert emitters along the header line to spread the water along the wall. The pump gph should be greater than adding up all the gphs of the emitters for all emitters to work.
Good Luck. Aloha