This is just me, but I'd get a set of 9mm carbide dies and familiarize myself with the press. You also need a manual, I did the PDF thing, had the copy shop print it off and put it in a binder with page protectors. That way I can also insert tips that folks post or I pick up along the way. Brian Enos forums is a wealth of info in this respect.
After that my next purchases directly 550 related were extra primer tubes, a casefeeder, and extra small parts:
Small parts kits - I bought a couple -
https://www.dillonprecision.com/20048
locator buttons - even though they are in the small part kit if you're like me you'll lose them:
https://www.dillonprecision.com/s000...ustomization=1
I bought a roller handle and used it for a long while then noticed that I was torquing the shell plate as the machine aged. I felt this was a source of some primer seating issues as well as case feeding and alignment with the sizing die, so I switched back to the original ball and the majority of my issues disappeared.
I'm also not into screwing dies in and out of the die holder so I bought EZ change kits for .40, .45 and 5.56:
https://www.dillonprecision.com/s000017 (didn't buy them all at once - LOL) I also purchased some upgrades for the powder measure:
Tube Baffle:
https://ballistictools.com/store/per...powder-measure
Micrometer Powder Bar Adjuster: Preinstalled:
https://benstoegerproshop.com/precis...-by-uniquetek/
User Installed:
https://benstoegerproshop.com/unique...loading-press/
The powder bar adjuster allows you to accurately record different loads without a lot of fuss - initially I was loading 9mm to plinking velocity as well as to power factor, so the micrometer made things a lot quicker.
Micrometer Bullet Seating Stem - same deal, easy to record and replicate:
https://www.cabelas.com/shop/en/horn...t-seating-stem
Everything else is just general reloading stuff.