I just leave my deep cycle trolling battery on a trickle charger when not in use, I think it's 0.75 amps. It's "smart" (pretty much all modern ones are) and it will kick off when the battery is charged, then when the charge drops below fully charged it will kick back on. If you don't have one like this I'd get one, only about 50 bucks at Crappy Tire or similar store/online.
Battery should be good for about 5 years. After that just recycle and get a new one. I tried to push it with my last deep cycle and kept using it past 5 years. One day I was only about 2 hours into trolling a small interior lake when the battery abruptly died. I had oars but it still took me about an hour to row to shore as a head wind has picked up. Lesson learned, don't try to push your deep cycle battery for too long. It's not like a car battery that you can just jump start and it'll get you through the day. If your d/c battery croaks you are SOL for the day. If you are out camping and have no way to recharge it with 110v power then buy a solar trickle charger. Spend a bit more $ and get one that'll charge the battery in decent time. Just leave it on the battery in the boat (I'm told it's fine to have it on the battery while discharging it when fishing) and point it right at the sun as much as possible. Should keep your battery topped up enough to fish for multiple days. When you get home toss it on the plug in charger.
Also while on the water you need a way to check your battery and make sure it has decent juice. If your motor is not too old it should have a battery tester button on it. On my Minn Kota there is a button on top by the throttle. Quick and easy to check what % the battery is at.
Last thought: I'd wait for a nice set up that has most of what you want. Seats, rod holders, and so on. This stuff is EXPENSIVE to buy new if you buy decent stuff. If the previous owner already bought all this stuff yes it will cost more than one that is bare bones, but not too much more, and it'll save you having to buy it all and install it on the boat. To put it in perspective on my new 12 foot boat I bought last year the seat mounts, seats, rod holders, oar locks, and oars ran me about $750 and I had to install it all myself. I added basic (hand crank) Scotty downriggers and a few cannon balls and that was another $400. Pay a bit more purchase price and find a guy that has already bought and installed all this gear.