Keep in mind that the GPH ratings on pumps is for when they're going down hill with the wind at their back. Taking into account head and friction, especially the feed tube has any bends, I think you should get a pump that's rated at about 1.5 times the gph you want to attain.
As far as turnover, 10x in between the sump and the display tank is really fine. Any additional can be added via a closed loop. And deciding on what additional gph you need is dependent on what you plan to keep in the tank. If you're doing an aggressive FO, with a Lion fish, etc, you don't need a lot of flow, so 10x out of your sump would be sufficient. You won't need any other flow sources, as long as the way to do the returns gives good flow coverage.
So I'd shoot one return pump rated at around 3,500, like the mag 36 for a submersible, or any of the nice external pumps. A forty gallon sump/refugium seems quite a bit too small for a 210g tank IMO, but its only an opinion.