The strong mount and bullet tray are worhtwhile options for the 550B. The standard lever arm is excellent, and I personally do not care for the roller arm. The low powder sensor might be worthwhile if you sit to reload. If you stand up, the powder hopper is right in front of your face.
Get a primer flipper tray, and it does not need to be the $13 brass ones that Dillon sells, and some extra primer pickup tubes. These also do not have to be Dillon's because virtually any primer tube will fit on top of the 550B's primer feed tube. (The primer feed tube, by the way, does not come out unless you want to change it for some reason. The pickup tube is placed on top of the feed tube and the cotter pin is pulled. The primers drop into the feed tube.)
A decent scale is an absolute must, and Dillon's Eliminator is about as good as any of that type but nothing special.
A stainless steel caliper is necessry, some kind of case trimmer is probably necessary, and if you trim a chamfering/deburring tool is necessary.
I'm not sure what Chico meant by the low-primer warning gauge unless he is talking about the low primer sensor that is part of the press. This is standard on the 550B. (Simple device consisting of a battery operated buzzer, a switch, and a plastic rod that sits on top of the primers in the primer drop tube. When the last primer is fed out, a knob on the rod contacts the switch and the buzzer buzzes.)
Buy Dillon carbide dies ($50) if you are loading for a handgun and forget about lubing and gaging the cases.
You may know this already, but I'll mention it anyway. While the 550B uses standard 7/8x14 dies, standard 3-die sets (carbide or otherwise) do not work very well in the 550B or any other Dillon. One reason is that the 550B does not use a standard expander/flare die but a funnel/expander/flare "die" that actuates the powder dispenser. (The 550B will not drop a powder charge unless a case is at station 2.) Another reason is that the 550B is designed to seat the bullet and apply the crimp separately. This means that the expander die of a standard 3-die set is superfluous, and you would need an extra die to handle the crimp operation. (Seating and crimping in one operation on the 550B, or any other Dillon, is not advisable.)
Some good, through reloading manuals are the Hodgdon #27, AA #2, Speer #13, Hornady 5th edition, and the LoadMaps. The Lyman 47th edition is also an excellent manual but is rapidly becoming dated.