I run the X30 at home, bolted to my chimney so it's at about 25 feet AGL. Key with VHF and UHF is not to use junk, thin coax. Go with at least RG8, RG213, LMR400 for anything over a few feet otherwise you're just heating up the coax.
Your manual will tell you the maximum draw. I suspect it will be about 15 amps. If you are going to add radios, I think you are right to consider more. Most people end up around 25A, which is enough for a 100W SSB or CW radio. If you plan to do any 10m or 6m FM at 100W, then you should look for about 30A continuous.
Astron supplies are made in Irvine, California.
Something to remember about car batteries is if you are inside, particularly if you're using it in a basement with gas appliances, is to use sealed batteries, SLA, AGM, etc. Regular unsealed batteries can vent hydrogen during charging.
"Silent enim leges inter arma." -- Cicero