[Hot Runr Guy]

Waxy,
from what I've read, the 36V systems seem to run a lot longer than 24V ones, so if you'd be using the Vantage all day long, go with the 36V version. When you add the 3rd TM battery, I'd re-rig the 3-bank charger just for them, and add a separate charger for the starting battery in the rear. You can certainly configure the wiring to give you 24V for the Motorguide, and 36V for the Vantage, it just depends on getting the wiring for the TM's run correctly. You may need to run new wiring to the rear, I do not know if Lunds wiring for both bow and transom are wired together, or are individual runs that terminate at the battery compartment.
HRG