First to answer one of your original questions: yes, I think East Penn makes the West Marine house brand, although I can't remember where I heard that, probably from the same guy in the bar who swore he had figured out a way to save money by re-engineering his diesel to run on coconut milk
I'd back up even one step further. What are you trying to accomplish? If you're just looking to basically put in new batteries for as little money as possible, use your credit, replace what you have, and relax and go sailing. If however you're looking at this as an opportunity to upgrade your system, then you have lots of options. First, go for as many amp hours as you can afford/fit in the boat. I've never met anyone who complained "I've got batteries that are too big," but I've met plenty of people who complained they don't have enough juice and have to run the engine too much, or run their batteries down too much, and the captain becomes an amp-Nazi ("turn off that light!"). So yes, you can do all the calculations for amp hours per day, and then double it, but you used the dreaded word: kids. Somebody will always leave a light on, or run the stereo, or a water pump, or whatever. And oem boats ALWAYS have battery banks that are way too small; it's one of the ways builders like Catalina save money.
I'm a big fan of the Slocum way of sailing: no electrical system at all, but in today's world, people want more and more electrical gadgets. So more power the better.
I humbly disagree with our fearless moderator about AGM's not being a cost effective solution. Yes, they have a higher initial cost, including a smart charger, but their long term cost per cycle is cheaper, and they have a much faster recharge rate (less fuel spent recharging the batteries at anchor). And they are more resilient to deep discharges, being stored without a trickle charge, etc. Conventional batteries usually give up the ghost sooner than they are rated for because most people let them discharge too far. And since your original question was about buying a new charger, you're already spending that money anyway.
I especially like the solar power response in this thread. Even a flexible solar panel that you put on the cabin top when you're not on the boat will keep your batteries topped off. Get a big bank(s) that can be used all weekend without totally discharging it, put the solar panel on during the week and your batteries will be topped off by next weekend. No running the engine in the anchorage for an hour on Sat. night to charge up the batteries and annoy your neighbors and yourself.
One more thing: use this opportunity to clean all the battery terminal connections, starter motor terminal connections, battery switch terminals, check your battery cables for size and corrosion, etc. Many of the excess current demands when starting are often due to high resistance from corroded connections and/or undersized, un-tinned cables. AGM or dual-purpose batteries should be fine for starting a small engine like yours if the system is properly designed, and then, again, you don't need "house" versus "starting" batteries.