The Enigma Machine and the Mars Sojourner Rover both have reductions to university research time. Since they recently had the Rover on one of the low-chance multi-chest sales, I'd be surprised to see either one offered as a flat 300-token deal. They know those are highly prized, so they'll make a lot more money selling them in a gambling format than as a guaranteed purchase.
There are probably a good handful of legendaries I'd consider spending 300 tokens on, but since I only have around 50 so far, it will be a while before I have to decide.