It seems like the choice of which and how many pumps will come down to noise tolerance. Is there living space in the basement where the sump might go? If not, and if you can put the sump down there, you could use something like an Iwaki that's bulletproof but loud. If you have to move a bunch of water (say, 7X turnover at 3,150GPH) with an under-stand sump, multiple (maybe just two) pumps might be better. You could run two DC pumps at low[ish] power to keep things quiet (something like a pair of Red Dragons, for example). In any case, the number and location of pumps will in part dictate your reactor options.
Let's say your sump is upstairs and you have two pumps, each feeding two returns. In that case, you could have a single Cerges (size 4 housing) on each one. No need to chain them. In fact, as long as you have multiple pumps, I think you're better off with multiple reactors too. Even if you can make a single reactor big enough, it would be better to have the co2 infused water entering the tank at multiple points.
If you're in the basement with a single, bigass pump, you could make a really tall Rex Griggs or a chained pair of Cerges reactors. I'm not sure how comfortable you are with DIY stuff like cutting and cementing PVC... Cerges can definitely be less intimidating if that's not your bag. For example, here's a reactor I made for a small tank. It's got a bypass (you'll need that to control the flow through your reactor) and was assembled entirely with threaded fittings:
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This sounds like a really big, and really fun project. I hope you start a journal once you get it off the ground!