Phil, you will want to use both the over flow pipes, not just try and use one and an emergency.
To do this, you will NOT use ball valves, rather, you will simple adjust the standpipe height so that 95-99% of the water goes down siphon no#1.
Then adjust the standpipe of no#2 so that only a tiny trickle goes down, but, if the 1# clogs or backs up for any reason, you still have 95% of siphon no# available for emergencies.
Not much will clog a 1.5" siphon.
Sump materials: Sponge blocks can be had from Swisstropical.com
Get the 4" thick 20 ppi size, this is plenty.
If you can use an inflow area for the degassing area that's sealed, then goes down under a weir, then back up, then you can us ethe filter socks, I'd use 100 micron 7" socks, say 2-4 of them.
Then the sponge, then the CO2/heater, filter return pump.
CO2: likely a Rio 2500 + their needle wheel and a larger Clear 4 x 10 or 4 x 20 filter cartridge housing to catch the mist, then simply have a loop in the sump filter area with the Rio upstream from the out flow(place this next to the return pump)
CO2: Dual stage regs, a 20 lb tank(buy 2-3).
Measurign CO2 rates: Use the inverted ml flask method. Measure mls of gas/min, or per 5 or 10 minutes, then you have a consistent FAR MORE accurate method for larger high volume CO2 dosing rates. Unlike Bubbles per second, this is a standardized method. I think I'm the only bone head to even suggest it in the hobby. But it works really well.
I needed to switch a regulator and CO2 system, so I did not want to have to tweak and toy with the new much, so this was a simple solution.
Relative pH drop is another.
The pH drop was exact when I used the gas mls per 5 minute method as the old reg that produced a wonderful growth rate. No significant difference.
I'd go with 3 and 5 mm.
You can also add a steel stainless wire mesh , a U shaped piece over the overflows to keep Amano shrimp and most of the smaller shrimp fry and smaller fish out and you can simply pull it off and knock the leaves off to clean it.
Ebay or a few folks here sell the wire mesh.