Gregory Adams

good that you're finally doing this, better late than never honestly. I went through the exact same thing with the rig choice and ended up pulling the 857 out of my go-kit after the first couple of activations. great radio but you're right about the power draw, especially if you're running it anywhere near full power on HF. ended up going with an ic-7100 for a while and now mostly use an xiegu g90 for the HF side of things. lighter, way more efficient, does everything i actually need in the field.
for power i run a 40ah lithium battery now but started with a sealed lead acid similar to what you have. the SLA is fine, just heavy. definitely put a load on it and see how long it actually lasts before you need it for real. nothing worse than finding out your battery is half dead at 2am during an actual event.
one thing nobody told me when i was building mine out -- dont forget the non-radio stuff. feedline, connectors, a roll of electrical tape, small multimeter, headlamp, logbook and pencil. i showed up to my first ARES activation with a beautiful radio setup and no way to write anything down. felt like an idiot.