Or does it mean i can buy 216 per day and not use it and let it accumulate to 1600 ba?
This one. I idea being you can buy 1600 BA before a NCB run without any large character active, that way the price per BA is very little.
with the cap of 160, is this the max amount you can have through regeneration only and not what you stockpile through purchasing?
Yes, 160 max from regeneration. After that you stop regenerating until you drop below 160.