In terms of batteries, I would think that upgrading the batteries sooner rather than later makes the most sense if you'll be boondocking quite a bit. The increased capacity helps even without solar, arguably even more so. It makes little sense to do anything until you figure out your electric requirements (at least roughly), however.
4x6V golf cart batteries is a much larger bank than 2x12V batteries, at least using the typical 12V RV/Marine batteries, simply because you have at least two times the total battery (measured in either weight or volume). Generally speaking, the 6V batteries are about the most economical in terms of useful capacity per dollar, but that's not necessarily true in every imaginable situation.