You don't need #18 wire for that mix of stuff, #22 would be plenty! The only precaution I'd take is to add a 3A fast-acting fuse at the transformer to protect the wiring in case of a short. The load you're describing is less than an amp, #18 is overkill unless you have it or are planning a significant expansion.
If you ran 100 feet of #22, and you had a 1A load on it, it would drop about 3 volts. That's considering a total of 200 feet of wire run, as you have the return path. Less load, less voltage drop. Obviously, if the run from the transformer is shorter, as I'm guessing it is, you'd have even less voltage drop in the wiring.
#22 is good for 5 amps enclosed, and you don't appear to be anywhere close to that. The bonus of smaller wire is it's easier to work with and conceal.
Here's an Ampacity chart for common wire sizes.