That does not seem like much water flow for this tank size my 600 has more flow. Are you going to use multiple pumps? Nice build. This is my plan in some amount of time. Cement seems like the way to go.
I applied the first layer of liquid pond liner (http://www.numa.sk/en/product/fluid-pondliner-skyblue-10kg). The second one will be grey. Doesn`t matter - there will be bacground but it is recommended to apply in different layers of contrasting colours, in order to see properly the coverage of every layer and not get lost in the job.