high current buck converter?
I have built plenty of high power buck converters, and a single-seat airplane powered by a large bank of 18650 cells. If I were you, I would not bother with a buck converter for this application. Here's some justification:


It's best for cell cycle life to avoid charging all the way to 4.2 V/cell. Stopping at 4.05 V / cell only costs you about 10% usable capacity, approximately doubles the cycle life, and reduces the time it takes to charge. It also brings your open-circuit voltage down to 16.2 V.
Most 18650 cells have relatively high internal resistance compared to, e.g. LiPo packs for hobby radio control models. Even with 20 in parallel, there will be considerable voltage sag under load. If you happen to use the same cells as I use in my airplane, the effective internal resistance of your pack would be about 5 milliohms, and you'll probably have another 10 milliohms in wiring and connector resistance which bring the voltage of a "full" pack under a 45 A load down to 15.5 V.
Most "12V" rated motors are intended to be used with automotive alternator + lead-acid electrical systems which run at 14.4 V. So you're only exceeding it's design operating point by 1.1 V or 8%, and even that will only be for a brief period until the cells get down to the "plateau" region of the discharge curve. These things tend to be built pretty rugged; it'll be fine.


You didn't ask about this, but - how are you intending to switch the power to the motor? If by a relay/contactor or a heavy-duty switch, you may need to take some precautions to prevent the contacts welding.

If you're using an electronic speed controller, provided that is rated for the max battery open circuit voltage, you can run it at slightly reduced torque or RPM command and as far as the motor's concerned it will be as if it were running from a lower voltage battery.