#4 wire to a rear battery should be fine IMO. Get a good deep cycle (there's a couple members here that sell nice ones for reasonable)....expect to pay @ 100$ for one.
#4 to that amp is also fine if you have a short run from the auxiliary battery. The only thing I'd really suggest to do with 1/0 gauge wire is the stock battery to ground and possibly the aux battery to the frame. Abour 4 feet should cover both so you're not breaking the bank.
If you have space for a large box there's no reason you can't get a single 18 to handle that power. IMO if you can't afford to blow the amp, don't run it at .5 ohm, stick to 1 ohm, it'll sound better and be much easier on it. If you can up your budget for a sub and drive down to around waterville/augusta way I can build you up a monster 18"