A couple of things. If you go to my web site and look under Alternators & Batteries, you'll find some information on isolators.
The main problem with cheap isolators is the fact they're just diodes, so the voltage drop can be as much as 1.2 volts under load., and a minimum of .5 volts. The way around this is to use one with a built in FET switch like the Hellroaring unit has. The FET shorts out the diode, and has just .1 volt of drop across it.
You can use relays, but that add a level of complexity, and about 4 times the headaches on the long run.
As for the battery, if it is inside the vehicle, you should used an AGM, and not a sealed SLI battery.
One last problem you're liable to encounter, and that's turning on the Check Engine light. Most late model vehicles read the voltage and current out of the alternator to adjust the injectors based on load, etc. If you suddenly connect a second battery that is at a charge level far removed from the SLI, the sudden change in alternator current will cause the ECC to turn on the Check Engine light. '