Some people like the 5 amp model because it is not too heavy and will run for 4 - 5 hours. Some people like the 7 - 8 amp batteries that will run longer than you are likely to walk anyway.
DO NOT USE ANY AUTOMOTIVE BATTERY... ever... All batteries are corrosive and can leak if damaged. Many automotive (RV, Boat, Tractor, Car..) batteries can leak if you just turn them on their side, and will leak very fast and soak you and your clothes with acid if you crack the case somehow.
Use a sealed gel-cell type battery, they are acidic as well, but it is a gelatin and will not do nearly the damage a liquid style battery will if you damage it. They also will never leak just because you tip it over. I found a great little stock of 8 amp batteries in the flashlight section of my local Academy Sporting Goods Store, they were only $15 each.
For a charger, I like using keyboard power supplies like those at Best Buy. They are relatively cheap and you can get them in 12 volt .5 amp to about 2 amps. That will refresh your battery easily if plug it in when you get back to your room. The only modification needed is snip off the plug end and replace it with a cigarette lighter end. You can also get 12 volt adapters at other places like Radio Shack and do the same thing for a small cheap charger.
If you do not stay in a room you can make a male to male cigarette lighter cord to charge the battery in the car, which also has the cool effect of running your accessories like the radio after you pull the keys out. However to charge the battery at all you should be running the car, otherwise your accessories will instead run down your fancy battery.