Just wanted to circle back on my experience since starting this thread. We installed a Shurflo Aqua King II 4 gpm variable pressure (up to 55 PSI/3.8 bar) self-priming diaphragm type pump. No need for an accumulator tank and took no time to install. After six weeks, we are quite satisfied with the performance. We do not expect it to last as long as the other kinds of more expensive kit mentioned in responses from various owners (though some say their Aqua King has provided many years of service). But that also means we can carry one or two spares aboard and still be ahead. Those spares will likely be 3 gpm capacity as per Bill Kinsey's suggestion. 

Ian & Margaret