Equalizing circuits are sometimes used in very large series strings where the DC voltage is 90 volts or more. This is fine during charging, but does nothing during discharge.
If you have a weak battery in a series string. During discharge, this weak battery will drop to a very low voltage even while the other batteries in series are still near full voltage. This causes the capacity of the series string to drop to the value of the weakest battery.
I wouldn't worry about it too much in a 12V system, as you will only have two 6V batteries in each series string. As long as the two batteries in series are well matched, they will share the load evenly during charge and discharge.
When I was sorting out all my used batteries, I fully charged each one and then let them sit unconnected for 4 hours. Then I took hydrometer readings on each battery. Then I matched batteries in pairs. Each pair had similar hydrometer readings. Then I load tested the whole bank (about 500 watts)and measured the voltage drop across each battery. It took a while, but after reorganizing the batteries a couple of times, I was able to match them up.
This is why it is usually recommended to buy all the batteries at the same time and in the same condition, less testing and swapping that way. But if you have the time, you can do it and end up with a good bank. I would suggest putting numbers on each battery so you know when they went into service and can keep records on them.
-Bill-