>>324729792
If you plan to host or practice by yourself the dlc is all worth it.
If you just want to turn up to lobbies you don't need any dlc whatsoever, only the host needs the dlc and the usual host has all of them.
Each dlc pack comes with 10 new tracks + 2 boss tracks* and a bunch of cosmetic stuff and extra campaign races. So if you buy one dlc you might as well buy all of them. 

*A boss track is a mega track made up of 5 normal tracks chained together with portals.