Cooling system - cooling system - cooling system...
The head problems you are referring to are those experienced with gaskets blowing and head body damage (warping, cracking etc). The head gaskets in these vehicles will probably always need replacing after a period of time - but if the head was last installed correctly and the car has been (and will be) looked after, then I would expect things to go well (exclusive of rocker/valve problems etc) for over 200,000kms (you can easily find Falcons at this stage that have not even needed the head to be removed).
Anyway, because the head is alloy, and the block is cast iron, the expansion/contraction rates (when heated/cooled) of these components are different. Therefore, it is possible for the head gasket to lose seal, or blow out, in which case you may find coolant entering the combustion chamber, or oil mixing with water (and vice versa), or both. Obviously, if the system isn't getting cooled properly, then the engine may reach higher temperatures, excacerbating this situation and perhaps even causing the head to warp or crack (very bad).
Your EA (unless modified) has a copper core radiator, alloy head, and iron block. Like similar engines, there is a tendency for corrosion to occur if the cooling system is not maintained correctly (correct coolant should be used - some coolants are no good for alloy/copper/iron systems). Go to <a href="http://www.discoveryauto.com.au/page4.htm" target="_blank">this site</a> for a little discussion of coolant types.
You must never mix coolants (when changing coolant or topping up), and you should only use distilled water to top up with (your tap water might be quite alright, but distilled water is quite inexpensive, so why not use it?). I have mentioned on these forums seeing an EF that had had it's radiator routinely topped up (and probably filled) with bore water - it needed topping up every couple of weeks (this should not happen!) - you can bet it had a blown head gasket at the very least - and if the condition was long term then no doubt the head would need some shaving/repair as well. Use only distilled water to top up - unless you are certain of the coolant type present in your system - then you may use a coolant/distilled water mix containing the same coolant.
Additionally, the inline 6 blocks tend to have a bit of casting sand in them (left over from the manufacturing process). This tends to circulate through the cooling system - and can build up in the radiator core. Of course, there is plenty of other gunk that builds up - so it's a good idea to have your cooling system power flushed - when you first acquire the vehicle (if you can afford - or unless you know it has been done) and then at regular intervals (I'm thinking about 80,000 for such flushes - and 40,000 for coolant changes is good - manual might say otherwise - please feel free to correct me on this).
That's my opinion anyway - and I'm not a mechanic (well, maybe a Sunday mechanic) - so I may not be absolutely correct in what I have written here - I'm certain other forum readers will feel free to pull me up on anything here!