As far as I am concerned, there isn't much you need as Hornady includes a great measure.
You will want a shellplate and die set for each caliber. You will want to be sure to have enough die bushings as you will find they are great. You will want to have a couple of extra case-retaining springs. You should have an RCBS Lock-Out die in station 3.
I like Hornady New-Dimension sizing dies, Lee or Hornady seating dies (or Dillon or Hornady if you are loading lead), and any good taper crimp die (for roll crimps, get the Redding Profile Crimp die).
I found that I liked the Hornady Powder-Measure and PTXs, the Dillon powder-measure and their "powder funnels," and the Lee Pro Auto-Disk and the PTE dies.
I never needed a case feeder with it since it is very ergonomic.
If you are going to load bottleneck cases, then you will want a 6" caliper, a 1" micrometer, and a method to trim your cases. You can simply use the Lee trimmer and the Lee case length gages with their Zip Trim. I have not ever had to trim a straight-wall case in 35+ years.
You will want a nice brush (a lady's make-up brush or a shaving brush) to keep things clean. You have to keep a progressive clean and the Hornady is very easy to keep clean.
If you plan to load a lot at one sitting, get 4-8 extra primer pick-up tubes so you have them all loaded with primers when you start.
I found the Lee Pro Auto-Disk powder measure and Lee powder-through dies to works great when developing loads for handguns.
You will want to get a little grease gun to keep it lubricated (and some 30wt oil for the ram) and a little on the bottom of the shell plate.
Handling the bullets and cases with my left hand was always easy, but that darn primer tube seems to run empty every 10 minutes or so.
What the world needs is an inexpensive way to quickly and automatically fill the primer tube.
You will want to get a "dedicated" wrench for installing and removing the primer seating stem under the press. If the threads are rough, you can find someone to borrow a tap to clear the threads (this is true for most things in life, it seems any more). I have a bad back and neck and I can not bend down and look at the seating stem so I have to do it by hand.
For any progressive press, you want the bench with no flex in it. Any flex will show up with primers not seating properly and bullets not being seated repeatably.