The 2560 has 16 analog inputs. You need 18 if you want to monitor all of your cells, so, a mux shield. But all you really need BMS for when bottom balancing is shutting off the charger, so you really only need to monitor one cell - the weakest one/quickest to charge.
Here is the relay I bought (but haven't used in my car yet):
G8P-1A2T-F-DC12 - OMRON ELECTRONIC COMPONENTS G8P-1A2T-F-DC12 - AUTOMOTIVE RELAY, SPST-NO, 12VDC, 30A | Newark element14 Canada