Mills have you decided upon the recipes you want to try?
http://www.alliantpowder.com/reloaders/default.aspx
http://data.hodgdon.com/main_menu.asp
Once you do you may want to get the primers, powder, wad and size shot you want to load so you are ready to go when it arrives. You will also need a bar for 1 and or 7/8 ounce as it comes with a 1 1/8 ounce bar. Be aware bars for single stage loaders, type 302, are different than the bars for MEC progressive reloaders, type 502. You'll also want a range of powder bushings, 27 through 38 will load just about anything in a 12ga.
Looking at the description you also need to be aware that the VersaMec and 600Jr are two separate models. One of the major differences is the type of wad guide they use, eventually you will have to replace a wad guide because the plastic fingers eventually break from fatigue. Good to have a spare hanging around. However in my opinion the older VersaMec is superior. It uses metal in places where the 600Jr uses plastic. If you need any parts from MEC you will need to be sure of the model you have.
http://www.mecreloaders.com/
This is not as daunting as it may seem. You are going to enjoy this when you get the hang of it.