Re: what do i need with my dillon 550
To improve the ergonomics a bit and speed things up: strong mount, bullet holder to bolt to it, and a roller handle, as already mentioned.
I like the Dillon pistol dies with larger entry radius, but am neutral on the rifle dies.
Extra decapping pins for the dies.
I do like their die rings and the wrench to go with them, and use their rings on all my other brands of dies, just to standardise and not have to mess with an adjustable wrench or vice grips.
One or more extra acro bins, to hold brass and then finished product. But you can get them at a good industrial supply/tool store locally.
Extra primer tubes are cheap, but to take a 3 minute break from reloading to refill the primers actually works out better for me, instead of having hundreds ready to go.
Caliber conversion kits for what you are going to load. But you may be able to save a little bit and just get a needed part (powder funnel or locator button), as there is some commonality, like same shellplate (#1) for .45ACP and .308 Win or (#2) for .223 Rem and .380 Auto. Take a look a the caliber cross reference chart and figure out what will end up being duplicate if you get all the caliber conversion kits that you think you need.
Get several extra toolheads and powder dies, so you can leave you dies set up where you want them. The you can change the powder reservoir around and just re-adjust it each time, and not have to mess with you die settings. Or spend the $, and get several quick change kits. Really depends if you want to do a few hundred of each round, or you are going to load a whole bunch, and then not do that round again for a long time.
I have not bothered with a case or bullet feeder. Once you get going, things move along fast enough without them, unless you really go whole hog and get a 650.
Order direct from Dillon, or find a few dealers like Brian Enos or others with free shipping, as they can't price the actual product below MSRP.
And then a case length gauge for adjusting the rifle dies. If you don't already have what you need, I like the Dillon over the Wilson.
http://www.rushusa.com/html/products/dillon/dillon2.htm