Here's what you need to go with that new progressive:
1) A good scale. Mine is RCBS balance scale. Works fine. Cheaper than digital.
2) Digital caliper instead of dial caliper. A great refinement over dial. Mine again is RCBS.
3) A log book for tracking your loading.
4) A toothbrush and canned air and lube to keep the press maintained. Probably an allen wrench set to remove and replace things.
5) No low powder alarm. You can see the powder just fine. Unless you are talking about a die that detects improper charges in the case itself. That would be a good tool to have if you have room for the extra die.
6) A pillow case or plastic bag to cover the machine when it's not being used.
7) A safe place to store the scale when you're not using it.
A bazillion bullets and primers
9) Trays for staging your bullets and brass. I use steel pie pans.
10) Maybe some extra tubes for filling primers if your machine has such a thing. I have 5 and prepare them all before loading 500 cartridges.
11) A box to store the finished rounds in.
12) A vibrating case cleaner.
13) A tool for separating case cleaning media from cases. I like the RCBS tumbling separator (about $20)
14) A filtration mask so you don't poison yourself when using the tumbling separator.
15) Disposable vinyl gloves for use during reloading
16) Good eye protection
17) Go-Jo with Pumice hand cleaner for removing lead from your hands after handling lead bullets.