While I might disagree with some of your choices I say go for whatever you want. It's your money and your tank. Do what you can and enjoy the ride. Buying new equipment down the road is part of the fun.
Nac9 I have and have no complaints. I use a 12000 return on my tank and would highly suggest something different. Definitely don't push claimed amount of water and you will have no extra flow to run a manifold for gfo carbon etc. I used 140 pounds of sand and have 1.5 inches of sand across whole tank so I think 200 is a little much. Take the money from that 60 pounds and invest in better return pump.
Just pick a return that matches your needs. You won't ever need to fiddle with the speed of the return. It stays constant. There's no point in having a dc return pump in that respect.
The sp4 is like 1500gph or so. Sp6 is 3000gph roughly. Personally I only plan on getting the sp4 when I get a new tank as I makeup the turnover via power heads.