First off, you DO NOT want a solenoid based isolator. You want a diode based unit.
My suggestion (similar to above) is:
1) Big 3 (done)
2) Biggest AGM battery you can afford in back
3) 200A diode based isolator (see picture below for reference)
You could do all of that, but its not necessary IMO. First off that 1608 is probably going to be
Next, if you want to run a two batt system, an isolator should be your LAST option. They are only used when you want to connect 2 seperate batt types. (ex. if you want to keep your flooded up front, and an AGM in back, then use an isolator)
The diode has an internal resistance, the standard voltage drop across a diode is 0.7V (basic electronics 101). Some companies probably use exotic materials which is how they might be able to reduce this voltage drop to the 0.4-0.6, but I wouldn't count on all isolators being that way without reading the fine print.
In perspective, with a 15ft run of power cable, pulling 100A (~1300W), a 0.7 voltage drop makes your 0ga run turn into a 6ga run.
wire resistance and voltage drop calculator
Some people modify their regulator on their alt to make up for this volt drop but I don't have any exp with that.
Your best bet, is to get 2 good batts that are the same size (and if possible have near the same datecode). Then wire them in parallel (w/fuses).
You can pick up 2 yellow tops for close to $300, then you'll only have to buy some 0ga to run to your trunk.
Personally, I would just buy one yellowtop for under your hood. Then when you upgrade, and if you have dimming problems, just buy a 2nd yellow top (same group size) for your trunk. If they are the same group size and wired in parallel, they act like one big batt and don't need isolators/solenoids.
