Welcome to the nano club, Jeremy.
Live rock is a definite. For a 5 gallon, I'd go with one pretty large piece to make a sort of island and go for 5 or so pounds for the tank... rule of thumb for live rock bio filtration is a pound per gallon. The more porous the rock, the better, and also the less heavy... heavy, solid pieces have less surface area and less chance of cool hitchikers emerging.
A lot of people use Eclipse systems for nanos. However, that wouldn't work for me because I live in an un-air conditioned home and it gets way too hot in the summer. I use an open tank, with the light fixture raised an inch or so off the top of the tank. This creates air flow. All summer, my nano tank stayed around 82-84, while some of my other closed tanks pushed 86 degrees.
You'll want a small heater and to take out ALL filter media in the filter. Sponges and cartridges that catch stuff in mechanical filtration are nitrate factories... and in a saltwater tank you shoot for ideally zero nitrate, but realistically 5 or under.
I use a biowheel on my ten gallon and haven't noticed any nitrate spikes, though a lot of people say they cause them. It'll have to be your call on that one, but I'll tell you I am in the minority, at least on here, for keeping biowheels.
Depending on how your live rock is arranged, the water flow from the main filter may be sufficient, but you can never have enough flow. I recommend in investing in a small powerhead, like the minijet. I currently use a microjet in my 1 gallon reef vase and I swear by the thing. It was affordable and puts out a lot of power for a tiny little powerhead. Also, the company has sent me free suction cups as spares.
For a substrate, you could go bare bottom, but I personally think that detracts from the tank. I use an inch or two of playsand in mine. Once again, a lot of people claim this produces too much silicates and leads to algae outbreaks. While I've had my fair share of algae, it's been nothing I can't live with.
Also you wouldn't need much so-called "marine friendly" aragonite sand for a 5 gallon. This stuff is coarser and most reefers and mariners use it in their systems.
Skip the "live sand" because your cured live rock will seed your tank and get it ready for a fish and some inverts.
Now for fish. On nano-reef.com some people actually put two small gobies in a five gallon, some even a clownfish, but I say that's pushing it, unless you're commited to very frequent water changes and water monitoring. And I say definitely pass on the clownfish (percula) as they can get several inches.
But there are many pretty gobies and other small fish you could pick out. Firefish are really neat, as are the purple firefish. But put some egg crate (light diffuser, the plastic grids under ceiling fluorescent lightstrips) over the tank if you go with an open hood as they will most likely jump out.
Inverts. You could easily and affordably get enough light to sustain soft corals. In my 1 gallon reef vase, I keep green star polyps and have kept zoanthids successfully. And I only use a ten watt 50/50 screw-in light bulb. This provides some ten watts per gallon.
You could swap out the bulbs in an incadescent hood with two ten watt 50/50s and have 20 watts over your tank, totaling 4 watts per gallon, and that's enough for most soft corals. You could also overdrive a regular fluorescent fixture and get enough light too.
Some soft corals are mushrooms, green star polyps, zoanthids, to name a mere few.
Moving inverts that would work are dwarf hermit crabs, shrimp, small fromia starfish, and probably a lot more I'm not thinking of right now.
Good luck!