Damn good price on Planes of Law. Congrats.

You’ll need the Planescape Campaign Setting boxed set to go with it, probably, although there will be a PDF available of RPGDriveThru if you just want something to refer to. It’s the glue that holds everything else together, but maybe not essential in hard copy. It all depends on whether you want to play, just read, or start a collection.

As a setting I found Planescape to be the hardest of them all to run (although I’ve never tried Dark Sun). I found the fonts ant the writing in general a pain to read for extended periods. I just couldn’t get my players to engage. They found it too alien and unrelatable, and were more suited to dungeon crawling, sitting in taverns and haggling with blacksmiths. We’re Scots. Half of them still wear kilts and carry broadswords at conventions. I love the art and architecture though, of course the illustrations and the visual feel is something the players rarely get to see.

I’ll let someone else answer specifics, because I don’t really follow this sub-forum much.
Anyways, welcome to Acaeum.