You can consider an Ariston 24l fan forced unit. G1 Gas in Centurion should be able to give you a decent price.
Dont go any smaller.
Remember the average shower head would be around 15l/min. Get a smaller shower head. Hans Grohe has their full range in a water saving option of 9l/min. You will be able to run 2x showers (18l/min( very comfortable. Only heat the water to the temp you require, dont mix in cokd water.