OK, I was assuming you got one and it was missing stuff, but understand you're getting a new one.
Nice optional features are the strong mount which will allow you to use their bullet tray. You can't really use their bullet tray without the strong mount.
The low powder sensor is unnecessary, just look at the see thru powder measure and see if it's low, if it is, add powder.
The allen wrench set is nice but the press will come with all the allen wrenches you'll need. If your workbench is messy and you're always digging around to find the right allen wrench, it's nice to have the strong mount mounted allen wrench set.
Get an extra tool head and powder die for each caliber you'll plan to use and swap the powder measure out from toolhead to toolhead.
The 550B comes with the low primer sensor.
I suggest getting 4 extra small and large primer pick up tubes, the press comes iwth one each already, so you'll have enough tubes to load up 500 primers with.
The maintenance kit is uncessary but nice. Dillon will send you any broken or lost parts for free. The advantage of the maintenance kit is you wont have the down time from when you lose or break a part, call Dillon, and waiting for it to arrive in mail.
The machine cover is nice as well but you can use a pillow case or big plastic bag.
I don't have the roller handle on mine.
In short: get extra toolhead and powder die for each additional caliber, extra pick up tubes. You can get the cover, strong mount, bullet tray, allen wrench set, extra powder measure for each additional caliber - down the road.
Here's what my 550B looks like:
One more option is to skip using their cheezy multi tool wrench to loosen or tighten die lock rings and get a big ol' socket with flex joint: