I had a Minnkota 80 on an 18′ glass boat for 3 years. 24 volts. I loved it.
After a few years of 24 volts, I was able to upgrade to a 36 volt/Kota 101 which you could consider overkill on an 18. Some 20’s & 21’s don’t even come with factory with a 101. The 36 volt allows me about about 3 more hours of time on the water prefishing. The 24 volt will offer you MUCH more power and longer time on the water than the 12. Especally if your in the current or wind a lot. I’d bet once you increase it you’ll never want to go back even with the extra encumbrance of a another battery.
The difference between 30% power on a 24/Kota 80 vs 30% thrust on a 36/101 almost knocked me in the water the first few times on the water.
Finally, I think the props on the 80s are about $15-$20 more than the 55lb thrust models. Might not seem like a lot but when you add it to any charger upgrade you might consider & $80 for another battery.. It all adds up.