Choice of pump really depends on if you want it in the tank or outside, and how much plumbing you're willing to do. The only really important factors are it being quiet and having a sturdy impeller that you can modify.
I've never regretted over engineering a system, but often regretted under engineering one.
Sorry to hear about your problems with the reactor. The Ista-mix brand (large size) one that I have dissolves CO2 perfectly and doesn't significantly cut down on the flow, though it's only on a 30 gallon tank (still, with the rate at which it drops the pH at only a moderate bubble count in my tank, I have to believe it would be effective even with a much bigger tank). Good luck getting your CO2 system up and running!
Thanks, everyone, for your suggestions regarding reactors, particularly Ista Max Mix CO2 Reactor. I set up a large size one submersed in the tank connecting it to a small submersible pump. Both are placed behind the driftwood, not really visible. Although I am running CO2 pretty high, it's dissolving nearly 100 per cent of it. Clear water, almost no bubbles. The drop checker turned yellow within 2 hours.