The questions you pose are all good ones. Unfortunately, there are no easy -- read simplistic -- answers, at least not any correct ones.
Let me address a few of them quickly.
Q: When does sulphation of the plates begin?
A: It begins at relatively high voltage levels, and accelerates as voltage drops. A fully charged battery leaving the factory will begin to sulfate, and as the voltage drops thru self-discharge or other means the sulfation rate will increase. Even a battery left on float at, say 13.2VDC for a flooded battery at room temperature, will begin to sulfate. That's why periodic exercising (deep discharges and full recharges) and equalization (raising the voltage to 15.5V or so for a 12V battery over several hours) usually have beneficial effects in knocking PbSO4 crystals off the plates and in bubbling enough to distribute the electrolyte more evenly and thus reduce the effects of stratification (different levels of acid concentration within the "layers" of the electrolyte, leading to differential corrosion, contamination, etc.).
Q: What can I do to reduce sulfation to a minimum?
A: Keep your batteries fully charged whenever possible, exercise them occasionally, and equalize them every few months.
Q: I let my batteries go dry. Will that kill them?
A: When you let the electrolyte level drop below the top of the plates, you are very likely to incur some damage. How much depends on a number of factors. All you can do is to add enough distilled water to cover the plates, fully charge, exercise, and equalize the batteries. And pray! Most likely, you will have lost some capacity.
Q: Should I replace my batteries if I think they're damaged?
A: Yes, BUT. There's no sense replacing your batteries unless you have examined your system setup and are prepared to treat the new batteries well. This means, at a minimum, that you have a 3-stage or better smart charger, properly sized for your battery bank; that your engine alternator has a smart regulator; that your wiring is of adequate size; that your connections -- all connections -- are clean and tight. If you don't tend to these things you'll be throwing good money after bad.
Q: Should I buy flooded, AGM, or gelled batteries?
A: IMHO, this depends very much on an analysis of the type of use intended, and your ability to care for the batteries regularly. Flooded batteries are still the price point, i.e., they are more economical than VRLA batteries. However, they require attention, they cannot be installed on their side or upside down, etc., etc. AGMs have lots of advantages, but some disadvantages, chief among these is their relatively low cycle life. Gelled batteries have greater cycle life than AGMs, but not as much as flooded batteries, and they are wonderful in applications where the boat sits at the dock with the charger on most of the time, with occasional overnite jaunts. I have some 12-year old gels in my basement which spent 10 years in service on a sailboat with this type of use. They still test about 90% good.
Flooded batteries can last a very long time, especially the pricey but very well built Rolls/Surette models. These can last 10-12 or even 14 years or so in heavy service on a cruising boat.
Q: Should I wait?
A: Maybe. There are several new battery technologies just coming to testing and to market. These hold great promise, if they don't turn out to be priced too high. Of particular interest to me is the Firefly Oasis microcell technology developed by Caterpillar; their Group 31 battery is going into full production next quarter, but initial sales will be to the trucking, military and other targeted sectors (not, unfortunately, the small-by-comparison boating market).
Sorry to have carried on so long, Joel. Hope I answered some of your questions. One strategy might be to commit, in your mind, to getting new batteries in the spring, just leaving the old batteries where they are. Spend the winter reading and learning, and planning your electrical system to support the new batteries in grand style :-)
Happy New Year!