I've also read about AGM's drawing too much power when charging after a deep discharge and overloading and possibly frying a regular alternator.
Possible, but unlikely, and besides, AGMs don't magically draw more power than any other battery. Most AGMs do have a slightly lower internal resistance than the same size flooded battery (it is the resistance of the battery and the wire which determines how much power will flow from the alternator though the battery), but it's not really enough to make any big difference.
Alternators are "current limited" - they won't produce more than they are rated at. They also usually have a "duty cycle" rating and are generally not intended to put out their full power 100% of the time or they'll overheat. That problem of potentially overheating the alternator can happen with any deeply drained battery - not just AGM. It pretty much won't happen with just one aux battery. Now if you had 4 aux batteries rigged into a single full-time battery bank, and that was deeply drained, you might have to worry about overheating your alternator because that battery bank would draw a lot of amps for a long time to get recharged.
Do you recommend Marine batteries? Are they really built better to resist vibrations?
Typically, cranking batteries have lots of thin lead plates. They are very good at dumping a boatload of amps in a bloody big hurry. But not for long. 600 or 900 amps for 3-5 seconds to start an engine is just about all they are good for. Draw too much for too long, and you've got to start worrying about those thin plates overheating and warping, then touching and shorting and bye-bye battery.
Deep cycle batteries typically have less number of plates, and the plates are thicker. They are very good at supplying less amps, but for a long time, and are far less likely to have the plates warp and touch.
You can use a deep cycle to crank an engine. You won't get as many max amps all at once as you would get with a cranking battery, but you won't hurt it. You can NOT use a cranking battery for deep cycle use however - you'll ruin it.
Typically, "marine" batteries are a compromise between the two. They have a medium number of medium thickness plates. They don't crank quite as well as a cranking battery, but they'll do the job. They can't handle deep cycling as well as a deep cycle battery, but you won't ruin it quite as fast that way as you would deep cycling a cranking battery.
AGM is just a lead-acid battery, but with fiberglass fabric sandwiched in between the plates. That is how Odyssey can make a "dual use" battery. They make a battery with a bunch of thin lead plates, which can crank really well, but that fabric prevents the plates from warping and touching, so you can also use it for deep cycling without having to worry about ruining it. Diehard Platinum - same deal.
Also, that fabric helps to keep the plates from moving, so an AGM battery is more resistant to vibration. Not that it would make any difference unless you spent a whole lot of time hauling *** down the Tanami Road (notorious washboard road in Australia). Even a regular old flooded battery has plastic separators to keep the plates apart - warping is an issue, vibration usually isn't. A flooded "marine" battery will have the same plastic separators. An AGM "marine" battery will have the same fiberglass fabric. So just because it says "marine" doesn't automatically mean that it can handle vibration any better.
In practical terms, the main advantage of having the fiberglass between the plates, is that it will usual allow you to pump a lot more amps through the battery - recharging it a lot faster - without worrying about warping the plates and ruining it. But normally, you need a really good battery charger to do that. A normal voltage regulated alternator is just going to charge it the same as it would any other battery.