A 55 gallon is a good size tank to start out with in this hobby.I do like a 75g. a little better because it gives you more width.
A sump is another tank/container that houses filtrations and more.Usually its under the stand.For instance on my system,I running approximately 55gal. sump on my 210g.The heaters,skimmer,prefilter(sponge),carbon bags,refugium and more is incorporated.Most people use them to add more water volume and hide equipment/filtration.You can run a successful system with or without one.
Best of luck.