I would like to add my opinions to this discussion:
I recently put a dual battery system in my 2010 SR5 Hilux. After alot of research and consideration, it consists of:
1) ARB dual battery tray
2) CTEK D250S Dual charger
3) Optima D34 yellow top Battery (55A-h)
I got the ARB dual battery tray because at the end of the day, there is only so much weight you can put on the wheel arch. There are some trays out there that say they can accommodate batteries larger than 100A-h. That’s a heavy bloody battery, and when you see they way the tray installs, just looking at it doesn't inspire confidence. Go for the known product, with the known backing, with the known reputation. I would strongly suggest you buy the tray from an ARB dealer, because if your wheel arches crack, I’m fairly confident ARB will help you out better than anyone else.
Forget about Redarc.! Their DC-DC chargers are aged technology and when you pay the dollars they ask for their products, you would expect to have the latest and greatest stuff. Also as some have mentioned, the Hilux alternator output is not enough to charge the primary battery, let alone an auxiliary battery to 100% state of charge, thus using a simple isolator is not going to do you any good for battery health. Batteries like to be at 100% state of charge, hence if you're already going to spend money on a dual battery system, I would strongly recommend you get a DC-DC charger capable of this. CTEK has a strong reputation for battery chargers, and the specs and features of the CTEK DC-DC charger absolutely whoops the Redarc. I got the D250S Dual from
http://www.batteriesdirect.com.au cheaper than any online store was selling the Redarc BCDC1220.
BatteriesDirect is an authorised dealer for CTEK (I have been burnt before buying "genuine" products on eBay. Avoid it like the plague! What the emails floating around say is true: it's cheaper for a reason) and they are most definitely cheaper than buying it from any physical retailer/store in Australia. Plus, at the time of my purchase they offered free freight. I think they still do. You don't have to get the dual (with solar MPPT) and could save yourself some coin, but I did because I will use it in future to trickle charge the batteries when I’m away boating. Plus if you get the dual now, that means you only have to get a solar panel without a solar regulator, making the panel slightly cheaper. The advantage then with this is that your system is nicely integrated, where the CTEK D250S Dual will prioritise between the solar input and primary battery/alternator.
I got an Optima D34 battery because of its reputation and at a price I could not resist from a wholesaler who also sells to public at close to wholesale prices. But in general, any AGM battery is the go for dual purpose (deep cycle & cranking) use in 4wd'n. The Optima website will say that they can be discharged to 100% capacity, but be wary of this, despite them promoting this, this is really just marketing hype. You will kill ANY battery (Optima included) by discharging to 100%. Chargers like the CTEK will help maximise the life of any batteries used in deep cycle applications. To save coin once again, given what you were saying, you are more interested in cranking applications only, so you can get away with a standard lead acid battery. In any instance, check the CCA of the primary battery and try to get an auxiliary battery with a CCA in that ballpark.
These 3 items above set me back $790 in total. You will also need to get a small wiring kit for the CTEK installation, but JB Hifi or any car shop such as Repco or Supercheap can fix you up with an 8-gauge amp wiring kit that should do the job fine.
As someone has already mentioned, when it comes to cranking or jump-starting a dead battery, you can wire in any solenoid/isolator with a switch to connect the batteries in parallel. I had a SurePower isolator for this purpose, but with a recent safety recall, ARB replaced it with a Redarc smart isolator (don't shoot me for saying to forget about Redarc earlier!). All isolators pretty much work in the same way, so I’m fine with the Redarc in this instance. (Just note my comment earlier how buying from a reputable dealer will ensure you get looked after if issues arise...). This will set you back about another $170, and you will need to use heavier gauge cable for this install (probably around 2-gauge) so if having another isolator is not an option, why not use the charger to ensure your auxiliary battery is charged to 100% capacity, and then IF required, use a pair of jumper leads to get the car started. Just an idea for the interim.
When it comes to battery monitors, my opinion is to keep it simple. Most (not all) are just fancy voltmeters with red and green LEDs. If you want a proper battery monitor, look at Victron and Sinergex, these come with shunts required to accurately measure the battery's state of charge, but expect to pay $400-$500 for one of these. I don't have any type of battery monitoring installed, and in your case, I would just keep a voltmeter handy. As a general rule of thumb, when no load is being drawn and the batteries are not being charged, any voltage around 13.5V indicates a fully charged battery (Green LEDs), and a voltage around 10.0V indicates a fully depleted battery (Red LEDs). Batteries due to be changed will struggle to hold a constant floating voltage, often not reaching above 13.0V, and this is the warning sign that your battery is on its way out and will need to be replaced soon.
Hope this gives you some guidance in designing your dual battery system.