Yes, there’s a main story event in the DLC with the single best piece of equipment - the Professor’s spectacles. Buy them, it’s your only chance!

Also there is some missable witcher gear in the very same quest, I think. Just get everything in the Borsodi Brothers’ venue.

I pretty much only play it one way every time… When I’m high enough level, I get Griffin Witcher gear, and when I’m high enough for Feline Witcher gear I stay with it through the base game and both expansions. I did go Bear once and it was okay but never felt as good as a set.