I've been trawling through various websites and blogs, and still feel like I could do with some guidance.
I have a dual battery system set up in parallel with A, B, A+B, Isolation selection switch.
Defender is kitted out for overland with fridge, led flood lights, winch and inverter.
I need to replace the batteries.
Does AGM really suffer with overcharging? Is AGM that much better?
Will a deep cycle cope with winch use?
Does running a regular battery down harm the battery?
What battery, x2, should I go for?

The bits I know.
Running down a standard battery is much worse for it than running down a battery that is designed for it.
Why would you run the winch off the second battery? Are you planning on using when engine is off? I always run it off starter battery not leisure one, otherwise the split charge system gets a real hard time.
I run two Odyssey pc 1500 and have now done that in 3 defenders without any issues. They have fun fridges, inverters, lights and loads of other toys including a winch on each Defender.

https://www.tayna.co.uk/industrial-batteries/odyssey/pc1500dt/

I know quite a bit about dual batteries, I set them up myself in about 3 vehicles so far and I had all kinds of systems and did a lot of research.
You can use AGM both as a start and auxiliary battery, but you will have little added value.
You can have a normal start battery and put an AGM deep cycle as auxiliary.
For AGM deep cycle, the recommended charging voltage depends on the AGM type, it can be 14.V, 14.2-14,4. or even higher, and at float a little less.
Your alternator , if is not a smart alternator, should put out between 13.5 and 14.5 V. You need to measure and go for an AGM with higher charge voltage, to avoid overcharging. Perosnally I rarely seen older vehicles putting out more than 14V, mostly because of the age. Maybe new they were reaching 14.5, so get a multimeter and measure it.
Vehicles made after 2006 typically have smart alternator, and there the voltage is maybe 13.5 maximum even less very often.
It will help us to advise you if you can tell us what vehicle you have.

If you have a smart alternator, it is not a huge issue regarding the life of the battery but it will impact the size you need.
If you think you can get away with an 80 AMP deep cycle, considering it will not be full unless you top off with solar or shore power, you might need to redo the math and put in a bigger 90 or 100 AH battery, considering that it will never be full.

MontyStory wrote:

.
Does AGM really suffer with overcharging? Is AGM that much better?
Will a deep cycle cope with winch use?
Does running a regular battery down harm the battery?
What battery, x2, should I go for?

Pretty much everything explained about dual battery systems here.