You don't say what your usage pattern is. The requirements for a "weekend" sailor are very different from a long distance cruiser or a liveaboard.
If you use your boat at weekends or for 2 or 3 days at a time with perhaps a longer period away once or twice a year then a system such as I suggested originally, perhaps with the addition of solar is more than adequate. The starting point is to do a usage audit of your typical consumption. The biggest power user is the fridge then when sailing the autopilot (and radar if you have it and use it). Other potential big users are navigation lights at night and anchor light then interior lights and entertainment. There are many ways you can reduce consumption such as extra insulation of the fridge, keeping it full with really cold things like frozen bottles of water and chilled beer; LEDs for all lights, minimising load on the autopilot by trimming sails well etc.
In my experience in this pattern of usage in both the UK and the Med is that daily current draw is in the region of 60-80AH. A battery bank of 270ah discharging to 50% (best for long life) gives a safe capacity for 2 days without recharging. An hour engine running will put back at least 20ah, solar in your environment probably 10ah increasing safe capacity by another day. In reality most people motor more than one hour a day if they are on the move every day, so capacity really only becomes an issue if you spend long periods at anchor in which case you do need to either increase your bank capacity or solar/wind or both. The reason for working on 50% discharge is that although AGMs take charge quicker, actually getting them back up to 100% is very slow. So work on leaving 50% and concentrate on getting back as much of your consumption as possible each day.
As for shorepower charging, there really is little need for charging the start battery as it should always be fully charged so if you can't get, or don't want to spend the money, a proper multi output smart charger, just get a simple 10a single output for your AGM bank. If you sail at weekend and then leave the boat on shorepower for the rest of the time, the house bank will be fully charged. In fact if your pattern of usage is as suggested above it will be fully recharged in less than a day. Higher output chargers only make sense if you stay on shorepower while using things like microwaves, hairdryers etc through an inverter plus the fridge when you want the charger to keep pace with the increased consumption. However in reality it makes sense in these situations is to use 220v direct. A 10A will keep pace with the fridge and lights if you have LEDs.
It really is worth fitting the BEP cluster. The basic Bavaria system is poor by today's standards with just the one isolator in the negative feed and the diode. You can leave the isolator in but in the on position with the handle removed. The cluster costs under £200 and gives you a VSR, independent isolators for each bank and a parallel switch. I have fitted 2 of these, one in my old 37 which is much like your boat. It is easy to wire because it can all be done in the battery compartment, although it makes sense to have the switches on the outside.
AGMs have a very low self discharge rate so you only have to think about how you replace your usage. in the last 4 seasons with my 33 that has a 270ah house bank, I have never got remotely near running low on capacity, even though I always stay at anchor and run the fridge all the time. I put the shorepower on when I get back but it normally only puts out 3 or 4A which means it is just topping up the last bit, and fully charges in less than a day. Over this winter from November until now I have put shorepower on only twice as the bank has never fallen below 12.7v
Hope this helps
