First, it may be tough to find a pump that will only turnover your pool once in 24 hours. That is just 18 GPM, that means at high speed it will be 36 GPM. Unless, your head loss is very high, you will likely exceed that. The smallest two speed I am aware of is 3/4 HP. The 3/4 HP Whisperflow will produce 70 GPM @ 50 ft of head on high speed and 35 GPM on low. Twice what you have specified. However, two turnovers in 24 hrs is probably better anyway.

Also, you will still need to keep a some residual CL in the pool even though you will be running 24/7. With two turnovers, you don't want to have to wait 12 hrs before a bug is killed. So, you will still need some CYA to hold the chlorine during the day. How much is debatable but I would have at least 20 ppm. Trial and error may dictate how little you could get away with.