You do not *need* a sump, but they provide numerous benefits.
Increased water volume (stability)
A place to hide equipment (skimmer, heater, etc)
A place to cultivate macroalgae for nutrient export
A place to grow live food such as copepods, amphipods, mysid shrimp, etc.
There are not many true plants for marine aquariums other than seagrasses, but there are tons of beautiful macroalgae species available.
I wouldn't advice a snowflake eel in a 55, especially with small fish such as gobies. While snowflakes primarily prey on small crustaceans, they are not above eating fish. They are also incredibly strong escape artists. Lid must be weighted or otherwise secured.
As for straightforward no bs advice... Slow. Best word of advice I can give. Read every day. Browse the forums and read from start to finish, any thread that catches your eye. Read up on every type of fish, algae, coral, or motile invert that you are even thinking about contemplating to dream of buying.
Don't rush the cycling process, or skimp on equipment. Get what you will need before you need it.
And if you don't know something, ask! We're happy to answer!
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef