A 55 is a common size and what I consider to be a starter tank for
SW.
You might want to read the
New Tank Setup List thread and some of the other articles;
Sumps Explained and the others in my sig below.
A 55 will allow you to keep 5 -7 small to medium size fish. You need to decide if you want a
FO,
FOWLR or reef tank.
IMO a
FOWLR becomes a reef tank in about a year. At that point you have the experience and set up to start goig reef, so why not <g>?