You are in the right place now, awestruck! and seriously, all throughout your setup procedures, check all "outside" advice (non-RC) here on the forum before you implement it. You'll get lots more points of view, and some potential bad advice can/will be avoided.
I'll answer what I can from your list...
1) What the tank's previous owners are probably referring to is that you would use one of the overflows for your drain down to your sump/fuge system, while the second overflow is simply used to feed a second pump which is only being used to move water. This second pump would suck water down the outflow pipe in the 2nd overflow, and then blow the water back into your display tank via the inflow pipe of the 2nd overflow. Does that make sense? While this setup works fine, if you plan on keeping high-flow animals like SPS corals in your display, you may need additional water flow. I say this because the drain pipe of the 2nd overflow can only drain so much water per/minute, and if you want a pump that jams out more GPH than the drain can provide, you've got a problem. That being said, I have an "over-the-top" closed loop (CL) system that uses a Sequence pump and a homemade PVC manifold that sits on top of my tank's frame, with 9 different nozzles pointing down into the tank. They aren't hard to build, and other than the pump cost, they're cheap. If this option interests you, ask away!
2) By using CL(s). Ex., my display tank has no equipment in it. No powerheads, no heaters, etc. It has an intake pipe for my CL system, which has become covered in coraline algae and blends into my rockwork. Since my CL output comes from the nozzles pointing into the tank from above, I don't have any equipment stuck to the walls of my display tank.
3) If you want to stock chaetomorpha macroalgae in your sump or fuge, you may want to add a powerhead to the tank w/ the chaeto in it, as it seems that chaeto likes to be continuously "tumbling" in the water current. Otherwise in my experience it just dissintegrates a slow death. But all of your water movement can come from the water draining into the sump/fuge from your display and being pumped back up to the display via your return pump.
4) sorry, don't know.
5) Yes, you've got it right. You can set up your RODI filter w/ an auto on/off switch system that activates the filter when needed, but in this respect I feel that simpler is better, aka less prone to break and make a mess. BUT, I have flooded my kitchen's tile floor 3 times because I forgot to turn off my RODI filter while filling a 5G bucket w/ FW.
6) sorry
7) definitely check these out. I just got one and love it! Pretty easy to set up and maintain, and it really, really helps combate cyanobacteria growth. My phosphate reactor has been a life-saver.
8) I would think yes, the support poles would suffice, but you sure don't want to mess up that calculation. Get more/professional opinions, and post pics if you want more precise advise.
Good luck!!!
-Steve