I love the 84"x24"x24" foot print. (Only if that is your foot print you have a 210 gallon, not a 160.) I wish I went with this instead of my 96"x24"x24". IMO the flow characteristics are much better in your system. You could easily use 2-MP40s for circulation. That is what I use, and with them at 100% they can barley do the job. Also your return pumps will provide added flow to the sides of the long tank.
For your ATO I would recommend the Tunze Osmolator, I've been using this system for well over a decade, and its been relatively trouble free. However I am on my 2nd unit. But now that you can pick whatever pump you would like as your ATO pump it is a much better system.

This set up has tons of potential! Especially since you are going with MH/LED combo. You are smart to take it slow. Good Luck!