a main drain is good to have, two are required to prevent entrapment. I would not use flexible pipe underground, schedule 40 PVC is standard. The claims of most mineral systems vary from misleading to outright lies. I wouldn't have a Nature 2 system and would cut it out if I did. Many people here love their Salt Water Chlorine Geretors. I'd prefer a thicker liner to thinner - can company 2 move on that?

I'd also look for two skimmers and several returns. Pipes should be larger where the flows are joined.

What pump / filter? Heater? Lights?