Keep in mind the output of that power supply is 13.8 volts - do not drive 12 volt bulbs directly with it. What I would suggest is two things. About 4 diodes of a 5+ amp rating, wired in series, which will drop the power supply output by about 2.4 volts. Second, run the power supply outout to a terminal strip. Add 5 fuses of no more than 1 amp each, and break you light circuits into 5 sections, one to each fuse. That was, there is no single high current line on the layout, and there is no need for heavy duty wiring. Plus with multiple circuits yo can switch things on and off independently, and a single short won’t put a full 5 amps in one single spot of the wiring.
Perhaps the only change I would make is to use circuit breakers rather than fuses. But fuses are simpler and cheaper as long as you do things carefully so as not to blow them often.
Picture your electrical distribution from a single power supply as a large tree (from the ground up). Large currents and large wires are at the trunk, with small currents out at the individual light bulbs. If you depend on a single large fuse or circuit breaker, all the wiring must be robust enough to pass just under the trip current for the breaker. Yes, 18 gauge wire is capable of handling more than 5 amps continuously in open air or with reasonable ventilation. With 10 amps, you need 14 gauge wire (house wiring size).
But - if your breaker is set at 5 amps, every part of your circuit must be able to handle 4.9 amps continuously. The wire must not have enough resistance to limit the current to 4.9 amps if a short circuit occurs at the far end of your distribution. I like the quarter te
I suggest you read the (limited) customer reviews. The 5 AMP supply seems to have a power limitation of 3 amps or so, or it overheats.
The 10amp supply (Pyramid PS12KX or PS15KS) seem to have much better reviews.
I bought a couple of inexpensive PC/monitor power supplies off of e-bay (about $12) which I am going to distribute around the layout as needed. While they are rated at 5 amps, I don’t plan on using them at full capacity.
You will need a voltage regulator to reduce the output to 12 volts (or slightly less). There was a recent thread on that here, search for LM317 voltage regulator.