LATE TO THE PARTY (AXE FX II XL)

I've personally just loved everything that comes in it! I've tried a few cabs that people sell but always end up going back to fractal impulses.
 
buy the Ownhammer high gain essentials IR's, they are pretty awesome.
also buy "FAS cab lab", it's cheap and you can mix IR's in this software!
 
Definitely get Axe-Edit. It'll make your life so much easier.
You don't really NEED any extras, though I've heard great things about the Ownhammer and FAS cab lab as well. Maybe download some presets from professionals and see how they built their chains so you can learn from it. I know Periphery has some on Axe-Change, and Born of Osiris' Lee McKinney has his for sale on JST, along with Asking Alexandria's Ben Bruce. I haven't downloaded the latter two, but I'm sure they're good!
Also, make use of the X/Y functions and scenes.
Enjoy!