I started off with a 3 gallon and it turned out great. All you really need to start off is rocks, sand, heater and HOB
filter. Let that cycle while you get a light source. I would go with the coralife T-5 mini from Foster and Smith. $60 I believe. And probably a korila nano powerhead.
Refractometer is something you should get for a tank this small as well. For livestock you can go small species gobies. They are great little fishes. Pretty much all you really need to get started. I would set up an auto top off using a float switch (cheapest solution). Search around or I can try to send some pics of mine. Top offs are a real pain in a small tank, but using the auto top off is great.