If you're going to make a FO or a FOWLR, then a 55g is the wrong tank unless you are going to only get mall fish. No tangs, Angels, puffers, larger wrasses or anything like that which you will find in FO tanks.
That being said, it makes the worst reef tank - aqauscaping is impossible and lighting it isn't the best . At 20", PC's are too little and T5's/Halides are a little too much in most cases. if you have an extra 6", you're beter off starting with a 75g or at the very least a 72g bowfront.
if you're not going to do a sump, you're going to have a hard time keeping it as the majority of the equipment will be INSIDE the tank and in plain sight. So, pump, heaters, temperature or any other kind of probe will be right in front.
75g is the best start as you can put a sump underneath the stand.
HTH
Jim