It's good you confirm it. Because for me it's not clear your intention and goal you'd like to achieve.
Passive measurements/monitoring and even log results to SD it's only one feature.
In your initial post #1:
I want to avoid that the cells get out of balance with each other.
My first thought was to connect all separate cells to an Arduino (with som I/O extenders?), measure the voltages, compare them and act upon differences. (ie. warn me or something)
Your thoughts were - act upon differences. That is another feature.
BMS can do both task, Passive monitoring and Active balance individual cell. How to do it described in details in data sheet linked to MAX11068. Have a look and make your decision if you gonna make balancing.
Design of system, we already discussed, it depends also on your decision if BMS has to do something else (temperature monitoring, control lighting/sound devices etc).
If you decided nothing else to do, you can build project as CrossRoad suggest. Disadvantage of such approach is than you have to packed arduino in the plastic box altogether with relay matrix and power source, and never touch it when it powered up, because it would be "floating"
with battery voltage +150V.
It's, again, up to you to decide, but I recommend split high - low voltage parts by using analog optocoupler. Arduino would be at ground level, safe to touch, connect other devices/computers.
Link to schematic how to build optocoupler is also posted, plus application notes how to design one.
There is one question, have you check on your battery, if you would be able to get access to individual cells? It's not in some insulated packaged? Would you be able to attach wiring?