First, some thoughts:
As long as you are not using the built-in monitoring system the Insight would have (that probably needs direct cell access, and/or communication with the original?), and just need to feed the main B+ and B- to the system, then a typical BMS can work...but I would use one that doesn't use FETs for output, rather one that controls a contactor. It doesn't actually have to operate the contactor, but the signal it outputs for that can be used to provide a warning to you (or the system) that you should stop right now and check what the BMS sees as wrong with the pack that it wanted to turn off all power to protect the cells.
Cell testing, etc., there's a number of good threads for, about testing various kinds of cells. Stuff by Pajda, Dogdipstick, etc, that include specific testing equipment and such. I don't have any links ATM, but you can look thru their posts from their profiles to find the threads/posts about that, and some of the other posters in those threads have threads/etc of their own that also discuss these things and may be of help.
If the cells are well-matched, good quality, and are not fully discharged and not fully charged for each cycle, then balancing won't matter all that much, as they will stay balanced. The EIG C020 cells I use are older than your cells, and still stay balanced on their own without any BMS...but I am using them well within their limits, not pushing them hard.
Balancing itself is just keeping them all the same voltage at some point in the SoC curve; usually that's the full charge voltage, or whatever voltage the BMS (if programmable) is set to do so at. It does not change whatever capacity/etc the cells actually have, it only helps you use more of it as the cells age and become different from each other.
If you have to use balancing to keep the pack functional, then the larger the differences between cells, the higher the balancing current will have to be to keep it balanced in short charge sessions. The lower the balancing current is, the longer it will take to balance cells with large differences.
Passive balancing just connects resistors across each cell group as it exceeds the balance start voltage limit of the BMS, and disconnects them as it drops below that.
Active balancing varies in exact method, but is essentially what I described before.
You can buy separate balancers in either type, that aren't part of the BMS, if necessary, but if you're using good matched cells it won't be, until aging unmatches them eventually.
Some of the BMS are also programmable for many limits, and have BT to talk to an app on your phone or anohter device, and some have dedicated screens that can connect to them wired (or sometimes wirelessly) that can be mounted in the vehicle for monitoring. JBD, ANT+, and some like DALY, but you have to be careful buying any of these because they're so popular there are probably counterfeits out there that aren't built / designed / etc properly....
Physically building the pack, I recommend that if you can you keep the cells in their original modules that provide stack compression and protection, even if you change the interconnections for a different number of series or parallel cells, or wire around dead cells, etc.
If you have to take the pack apart / put it into a different enclosure, I recommend building it to compress the large faces of the cells thru the stack; there are a few threads and posts about this scattered around the forum.
PS: I wish I had access to those stacks of cells....
(just a small stack would be enough :lol: )
