I realize you are asking for advice from Disco II owners who have installed a dual battery system, but you also asked about reliability and the ease of use for a major brand like IBS or National Luna.
I am not a Disco II owner, but I do have an IBS Dual Battery System installed in my Classic so hopefully me putting my two cents on the IBS system won't be out of line.
The IBS system is not too complicated to install, very easy to use, and so far has been very reliable, which I am grateful for because I have had to put it to the test on a few occasions.
When my main battery was so dead my dash clock wouldn't even run, I just pushed the link button to manually link the batteries and it fired right up. I then pushed the link button to put it back into the normal mode and the main battery was back up to full charge by the time I got home. If however I did forget to switch back in to the normal mode, it will automatically change it back after thirty mins so there is no need to worry about causing a problem there.
There is also an alarm that beeps if it detects low voltage for either battery. I have used my auxiliary battery until the low voltage alarm went off and just like when my main battery was dead, it charged the auxiliary battery back up by the time I got to my next stop. The only real gripe I have with it, is that I wish it showed a little higher voltage level than 12.8v on the display, 12.8v is adequate but a little higher like the National Luna's would be nice. That would mean a bigger display though, and the compactness of the IBS display makes it easier to put into tight spaces in the interior (kind of a catch 22 I guess).
All in all, its very intuitive and easy to use. It takes care of everything without having to stress and anyone could be trained to use it in under two minutes.
My set-up is the factory Lucas alternator, the IBS Dual Battery System ($380 w/full wiring installation kit), the added Relay Booster Module ($85), two yellow top Optima D34's ($155 each), and two universal battery hold downs ($7 each). You can pick whichever mounting suits you, I chose the swivel mount ($20).
I would recommend the IBS system highly, but as is the way with every decision, to each their own. If you want a little more info on what the installation is like, I took pictures when I installed mine (post #'s 204, 205, and 208 in my build thread), plus I tried to add a little more in-depth info for how it all connects up. I know its not in a Disco II but the installation of the wiring should very similar, it is just the locations of the specific parts that may be a bit different.
Here is the link:
http://www.expeditionportal.com/for...tomization-Of-My-88-RRC?p=1190309#post1190309
P.S.
It is a stand alone system, so there is no need to mess with any of your factory wiring, you would only be dealing with the wiring necessary to install the system by itself.