When I switched from 6 volt Trojan T105''s (still a great bargain for a flood cell) to AGM''s I didn''t redo my charging system one bit other than to make a few adjustmkents on the charger itself.
It already was a good 3 stage charger. They are just a flat out good idea period - no matter what battery system you use. They charge faster, safer, and yield longer battery life. They save you money by giving you more hours! The batteries go dead eventually - all of them.
I also suggest you not get suckered into exceptionally high rate alternators. They generally have a poor payoff. They only run real high for a VERY short time before a smart regualtor will knock the voltage down. You can''t throw huge amp loads at a battery all at one time for a long time. When you start running the numbers you find a high output alternator shaves only a few minutes off your recharge time. they also eat belts faster and cost a whole lot more.
Charging systems first, then batteries would be my order of preference.
I just added a Xantrex Link10 battery monitor. That is one slick device for telling you about how you use power and how much you really have at any point in time. If you shop you can find them for about $125. It can be added to any single bank system. After a few trips with it you''ll know more about your personal battery consupmtion to resize it perfectly.
You want all the power you need, but you only want to pay for all the capacity you''ll really use.