A few months ago I picked up a used Dillon 550 locally, only took a couple years of waiting for one to come up. It was part of an estate and the guy had no clue what the press was or was capable of. I picked it up for a good price as it had the strong mount, two tool heads, two powder dies and a couple conversion kits. Over the past several months I have grown to love the press, so has my wife now that ammo is "flowing" from the press.
Having been through this here is what I did. I brought the press home and mounted it, I already knew where it was going. Next I stripped it completely to the frame, every nut, bolt, spring and tube! The previous owner took care of it, but it hadn't been used in some time and needed a good cleaning. This showed me how the press worked AND got the press in good working order. While doing this I kept track of what I had and didn't have...primer pickup tubes and the small primer magazine tube were missing. When I had done a mock-run for 9mm and 5.56mm I found some other non-essentials (parts kit, etc.) I wanted and included them on the order list. Order a tool head for each caliber...set the dies and leave them, even if you move the powder measure. I am fortunate to have one setup for 5.56mm, one for 9mm and a third sitting on the shelf for whatever friend wants to make some ammo.
Once you have the parts and are ready to go, do as the others have said...GO SLOW! Trust me, I cleaned that press 3 times due to spilled powder! Turns out a bit of surface rust on the inside of the powder funnel will catch on a 5.56mm case lip and send powder everywhere...3 times before you figure it out. Some 600 grit sand paper and saying "serenity now" while buffing the powder die cured that issue. Get the feel of the tension needed on the shell plate, if there is too much the ball doesn't click well and too little where the shell wobbles...this also was an issue helping me spill powder.
I don't have the low powder alarm, brass feeder or other add-ons. I like the standard ball handle, I feel it helps give more feedback as to what it going on. Between feel in the handle and sound you can tell immediately when something is going sideways. Definitely put lights on the press, I did it via led strip lighting bought on Amazon which let me cut and solder pieces together to get exactly what I wanted for lighting. Watch YouTube videos when you have questions, there is a video for everything about the press.
When loading, never talk with anyone...it just doesn't work. Never leave the shell plate "half-full", run it empty and start fresh, it only takes a minute to keep things straight and seconds to screw them up! I've loaded a couple thousand rounds over the past couple of months and really enjoy the press now that I know the sounds and feel the press uses to "talk" to me. My production rate is probably well below what Dillon says the press is capable of, but I'd rather keep my fingers, eyes and weapons than have a higher production rate.