First there are couple things to be aware of.

1) There is no physical limit to the flow rate in pipes. The numbers that you often see are recommendations and not absolute limits.

2) The flow rates posted on filters are maximums, not minimums and in most cases, filters do better at lower flow rate.

3) Forget everything you learned about turnover. It just doesn't mater. You can run your pump as short as you like as long as you get enough chlorine in the pool. I currently run my pump only 4 hours per day and more than half of that is on low speed so far less than one turn per day. Every pool is different but most pools can get away with less than 4 hours per day.

As for pump size, you did not mention any water features so there is really no limit on how small you can go. I have a 1/2 HP pump on a 20k pool as an example. Smaller pumps will filter water better and cost a lot less to run. However given the pump height relative to the pool, you might stick with at least a 3/4 HP. As long as you have priming valves or check valves on the pump, you should be ok.

As for two speed pumps, yes they can save quite a bit in operating costs so that is something I would highly recommend.