2nd what Aaron said. You will never be able to turn a 21ft. boat with a kicker they way you want. Do not get the panther system. Steer with the trolling motor is the best way. Also way better to get a tiller and install a trollmaster pro 3 for the speed regulation. It is the one with the remote. That way you don't have to sit in one spot to adjust the speed. Also does Mercury now have a fuel injected kicker? If so get that. I am disappointed with my carbureted Merc kicker. I have to run it dry every time I use it at the end of the day. If I don't it starts to run like shit. Reasoning for the extra battery is the power steering. The electric power steering pulls a lot of juice out of your battery. I know a few people that had to use their kicker to get back to the boat ramp at the end of the day because their starting battery was dead.
The 15hp Merc. on up is now EFI. The changes they made to the last 2-3 years of 9.9 and 15s (carbed) have been much better than in the past; but still not as good as EFI.
To the OP. Main reason for 5 batteries (as recommended by Mercury) is 1 for the main motor mainly due to the amount of juice that the power steering takes and how sensitive the electronics are in the motors these days (with Verados). The other batter starts the kicker and runs all electronics. Tiller vs forward controls is a horse a piece. I do 99% of my steering with the bowmount but on last boat I ordered a tiller for a couple reasons and will be on this years boat. 1 being cost (about 1200 less the forward controls) and the rare occasion I would want to hover over fish in reverse with the kicker (VERY RARE). You can still tie the kicker to the main motor if it is a tiller. You just have to go to the back of the boat to raise, lower, start, and put the kicker in gear. All other functions can be ran with the I-Troll or trollmaster and the bowmount.
And, since no one has mentioned it I will. You did chose the wrong boat. Get a Yar.... Just kidding.