They are hard to keep, not very hard. I had them for several years and if I did not move I'd still have them. (it was so much fun removing 100s of lbs of sand and catching all the eels to give them away). If you have any questions feel free to ask. Since you don't have the tank setup you are already ahead of the game.
A garden eel tank needs to be a garden eel tank first a reef second. Adding one or 2 to an existing reef is asking for trouble. They are pretty social too so I'd consider getting at least 4 and expect 3 to survive long term, maybe all 4. You want spotted garden eels (black and white) not splendid garden eels (the orange ones). While you can keep splendid they do not fare as well as the spotted. Can't recall the scientific names off hand.
Start with the eels then try and add other things later. You already know you need a deep sand bed, 8" is good. You could go a little deeper but that should be sufficient. You will not have room for a ton of live rock in the tank but having so much sand once it becomes live you will have lots of biological filtration. Initially garden eels are VERY skittish. They are afraid of everything including their own shadow. When you first start feeding them they will be spooked by their food. Live brine may be needed initially but start mixing frozen and move to mysis as soon as possible. You likely have to drop the food in from the side, or step far away after dropping it in as they will be afraid of you in front of the tank. I never tried feeding them anything but frozen food and live brine. They may go for pellets but mysis is a nutritious enough food to make up the bulk of their diet. LRS is another good option.
Flow is very important in a garden eel tank, the flow needs to carry their food to them. If they can't reach the food from their hole they will not eat. They do move from time to time but once they are in their holes you may need to adjust powerhead placement to ensure food is going to pass them. You also don't want them moving too much as this is the only time you really have to worry about them jumping, or the first 30minutes in the tank.
Lighting, corals, inverts don't matter to them. If you have for instance your pom pom crabs or cleaner shrimp they will startle the eels when they pass them but other then that they will not effect them. Initially I kept just the eels but once they are comfortable you can start adding other fish. Nothing too aggressive, I kept a file fish, copperband butterfly, and later even lyretail damsels with them without issue. This was only after I had them for years and they were comfortable. Adding just a clownfish or 2 after a few months shouldn't be an issue.
edit : here are some pics, you can see I also had a bunch of firefish in with them. I had all the flow going one way and the sand started at about 10" and slopped down. All the rock was contained to one side. I actually added more sand at one point too. I miss these guys...